WEBVTT

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now towards the end of the last video I

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mentioned that if we were going to fully
now towards the end of the last video I

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implement the header then we could get
mentioned that if we were going to fully

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our head class the same ability to add
implement the header then we could get

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tags and build up its contents and in
our head class the same ability to add

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fact we might create a special kind of
tags and build up its contents and in

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tag class that can contain other tags
fact we might create a special kind of

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and then have both head and body inherit
tag class that can contain other tags

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from that but we are keeping things
and then have both head and body inherit

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simple here and at this point we've now
from that but we are keeping things

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got all the classes that make up our
simple here and at this point we've now

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HTML document and we can create the
got all the classes that make up our

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document class let's go ahead and do
HTML document and we can create the

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that in the bottom
document class let's go ahead and do

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ok so....
that in the bottom

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....
ok so....

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....
....

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...
....

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...
...

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...this init method
...

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defines three data attributes to hold
...this init method

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the doctype head and body of a HTML
defines three data attributes to hold

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document now in all three cases a new
the doctype head and body of a HTML

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object of the appropriate classes
document now in all three cases a new

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created as you can see and each attribute
object of the appropriate classes

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is assigned a reference to the new
created as you can see and each attribute

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object so our HTML dot class is made up
is assigned a reference to the new

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of instances of the three other classes
object so our HTML dot class is made up

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so you could say that it's composed of
of instances of the three other classes

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the three other classes and that's
so you could say that it's composed of

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composition now this example is pretty
the three other classes and that's

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uncomplicated because the HTML. class
composition now this example is pretty

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is entirely composed of other classes it
uncomplicated because the HTML. class

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doesn't have any data attributes of its
is entirely composed of other classes it

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own
doesn't have any data attributes of its

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now that doesn't have to be the case so
own

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I don't want to give the impression that
now that doesn't have to be the case so

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composition means creating a class and
I don't want to give the impression that

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entirely out of other classes
composition means creating a class and

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the important thing is that our HTML
entirely out of other classes

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. class contains other classes and
the important thing is that our HTML

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can make use of the attributes that
. class contains other classes and

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those classes provide alright i'll
can make use of the attributes that

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carry on with the code
those classes provide alright i'll

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so we can see this program working and
carry on with the code

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how it produces a html doc so we
so we can see this program working and

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want to add a method....
how it produces a html doc so we

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...
want to add a method....

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....
...

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...
....

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....
...

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....
....

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...we need to
....

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add content to our document and HTML
...we need to

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content consists of various tags so what
add content to our document and HTML

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given the html. class and add tag or
content consists of various tags so what

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add_tagged method to allow
given the html. class and add tag or

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tags to be added to the document this
add_tagged method to allow

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method just delegates the job of adding
tags to be added to the document this

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a tag to its instance of the body class
method just delegates the job of adding

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the body class takes care of
a tag to its instance of the body class

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implementing the process of adding a new
the body class takes care of

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tag to its contents the HTML . class
implementing the process of adding a new

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just passes on the name of the tag and
tag to its contents the HTML . class

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the contents as you can see on line 53
just passes on the name of the tag and

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now we could have called this method
the contents as you can see on line 53

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anything add_content for
now we could have called this method

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example but by giving it the same name
anything add_content for

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as the class that will actually be doing
example but by giving it the same name

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the work for us our body class in this
as the class that will actually be doing

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case we can pass the HTML document
the work for us our body class in this

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object to any function that adds tags
case we can pass the HTML document

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now the other thing we want to do with
object to any function that adds tags

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html objects is display so i'm
now the other thing we want to do with

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going to add a display method....
html objects is display so i'm

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....
going to add a display method....

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....
....

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...
....

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...
...

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....
...

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....
....

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so once again this method delegates most
....

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of the work to the classes that contains
so once again this method delegates most

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so the DTD is displayed by calling the
of the work to the classes that contains

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doc type instances display method now after
so the DTD is displayed by calling the

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the DTD everything else is enclosed in
doc type instances display method now after

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the HTML opening and closing tags on line
the DTD everything else is enclosed in

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57 and 60 now we could have created a
the HTML opening and closing tags on line

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class to handle that but that's really
57 and 60 now we could have created a

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going to be too far
class to handle that but that's really

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there's only one html tag in a
going to be too far

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document and hard coding like this is a
there's only one html tag in a

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much simpler solution now the display
document and hard coding like this is a

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the remaining content is also delegated
much simpler solution now the display

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to the appropriate classes so apart from
the remaining content is also delegated

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including the opening and closing HTML
to the appropriate classes so apart from

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tag our documents display method
including the opening and closing HTML

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doesn't have a lot to do
tag our documents display method

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once again we could have called this
doesn't have a lot to do

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method anything but by calling it
once again we could have called this

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display it matches the corresponding
method anything but by calling it

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method in the classes its composed of
display it matches the corresponding

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by calling the method display it can then
method in the classes its composed of

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be used by any function that displays
by calling the method display it can then

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tag objects in fact the method it has are
be used by any function that displays

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exactly the same as the method in the
tag objects in fact the method it has are

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body class so our HTML dot class is now
exactly the same as the method in the

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displaying polymorphic behavior it is a
body class so our HTML dot class is now

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html doc but it can also be used
displaying polymorphic behavior it is a

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in place of a body object it can even be
html doc but it can also be used

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used in place of any of our tag
in place of a body object it can even be

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subclasses when pass to functions that
used in place of any of our tag

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just need to display method so we've
subclasses when pass to functions that

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implemented polymorphism without using
just need to display method so we've

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inheritance we've used composition
implemented polymorphism without using

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instead now the program itself does use
inheritance we've used composition

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inheritance all our other classes
instead now the program itself does use

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inherit from tagged but the HTML doc type
inheritance all our other classes

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isn't a subclass of tag it provides the
inherit from tagged but the HTML doc type

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same methods as their tags but does so
isn't a subclass of tag it provides the

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by defining methods with the same name
same methods as their tags but does so

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and then delegating the implementation
by defining methods with the same name

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of those methods to the classes that is
and then delegating the implementation

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composed of alright so let's see this
of those methods to the classes that is

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running to make sure that this theory
composed of alright so let's see this

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that we've been talking about does work
running to make sure that this theory

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in practice so we're going to add a
that we've been talking about does work

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method down here....
in practice so we're going to add a

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....
method down here....

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....
....

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...
....

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....
...

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....
....

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...
....

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....
...

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....
....

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...
....

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....
...

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....
....

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...
....

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...
...

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...creating a new
...

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HTML object and then were assigning the
...creating a new

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reference to it to a very variable
HTML object and then were assigning the

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called my_page so next we
reference to it to a very variable

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use this add_tag method to add a
called my_page so next we

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heading and a subheading now the add_tag
use this add_tag method to add a

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method remember just
heading and a subheading now the add_tag

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delegate to the body classes add
method remember just

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underscore tag method so these elements
delegate to the body classes add

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will really be added to the body section
underscore tag method so these elements

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so effectively we're building up the
will really be added to the body section

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document by calling the add_tag
so effectively we're building up the

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method to add the content that we want
document by calling the add_tag

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when we done the last line we're calling
method to add the content that we want

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the display method to display the
when we done the last line we're calling

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finished document
the display method to display the

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alright so let's just run the program
finished document

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need to put a colon in that line
alright so let's just run the program

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correct on line are 63 and run this
need to put a colon in that line

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now
correct on line are 63 and run this

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and we got an error over here and we can see
now

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that the typo looks to me that I've use
and we got an error over here and we can see

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start _ capital T so just click on
that the typo looks to me that I've use

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that to go to the link that's the
start _ capital T so just click on

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problem you can see added a capital T
that to go to the link that's the

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i'll make the lowercase T because
problem you can see added a capital T

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obviously the the attributes
i'll make the lowercase T because

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should be or have rather a lowercase T
obviously the the attributes

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in its name
should be or have rather a lowercase T

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let's just try running it again and this
in its name

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time we seem to fix it which is much
let's just try running it again and this

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better
time we seem to fix it which is much

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alright so you can see now if
better

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you familiar with HTML that this is now
alright so you can see now if

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perfectly formed HTML code
you familiar with HTML that this is now

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now it's not as readable as it would be
perfectly formed HTML code

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if a human it type it manually but line
now it's not as readable as it would be

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breaks are ignored in HTML documents
if a human it type it manually but line

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there only added to make it easy for
breaks are ignored in HTML documents

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humans to read if you're not familiar
there only added to make it easy for

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with HTML the real test will be if it
humans to read if you're not familiar

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displays correctly in a browser so let's
with HTML the real test will be if it

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actually make a change so that we could
displays correctly in a browser so let's

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write the document out to a file instead
actually make a change so that we could

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of just printing it now the only changes
write the document out to a file instead

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we need to make are to the three display
of just printing it now the only changes

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methods i'm going to start with the base
we need to make are to the three display

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class the tag classes go back up and
methods i'm going to start with the base

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look at that
class the tag classes go back up and

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so the display method here on the line
look at that

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11 we want to change that what we're
so the display method here on the line

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going to do is add another parameter here
11 we want to change that what we're

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so after the display itself...
going to do is add another parameter here

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....
so after the display itself...

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....
....

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ok so we've added a named parameter called
....

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file that matches the file parameter of
ok so we've added a named parameter called

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the print function so we need to
file that matches the file parameter of

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do the same in the body class down here
the print function so we need to

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under display in the body class on line
do the same in the body class down here

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38....
under display in the body class on line

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...
38....

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....
...

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alright so we added a filename parameter
....

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as you can see there to this method as
alright so we added a filename parameter

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well and then provide the default value
as you can see there to this method as

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of none so if a file isn't specified the
well and then provide the default value

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class will be printed to the screen
of none so if a file isn't specified the

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effectively alright so now the
class will be printed to the screen

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display method of the html. class
effectively alright so now the

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ok so down here in the display method
display method of the html. class

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we put....
ok so down here in the display method

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....
we put....

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...
....

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....
...

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...
....

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...
...

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....so now that we've done that we can
...

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now send the output to a file if we want
....so now that we've done that we can

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to using the technique we saw back in
now send the output to a file if we want

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Section 10 when we're writing to files
to using the technique we saw back in

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so let's end this video here now in the
Section 10 when we're writing to files

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next video we'll come back and test
so let's end this video here now in the

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this will talk a little bit more about
next video we'll come back and test

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composition and then we'll move on to a
this will talk a little bit more about

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challenge see you in the next video
composition and then we'll move on to a