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<P><FONT SIZE="5">2.6 Library Macros</FONT><BR>



<P>FunnelWeb provides a library macro feature that allows
you to redefine macros. Normally, FunnelWeb will generate an
error if you attempt to define a macro of a particular name
more than once. However, if you attach a different number of
<SAMP>@L</SAMP> markers (up to five) to each definition,
FunnelWeb accepts the multiple definitions, and at tangle time
uses the definition with the least number of <SAMP>@L</SAMP>s.
For example:

<P>
<PRE>
@$@&lt;ugly duckling@&gt;@L@L@{egg@}
@$@&lt;ugly duckling@&gt;@{swan@}
@$@&lt;ugly duckling@&gt;@L@{signet@}
</PRE>

<P>In this example, the ugly duckling macro will expand
to <SAMP>swan</SAMP> because the swan definition has the least
number of <SAMP>@L</SAMP> markers (i.e. the lowest library level).

<P>No two definitions may have the same name and level, but
definitions having the same name, but differing levels, are
independent of each other and can have different call number
constraints. They can even be defined additively,
with their multipart definitions interlaced. For example:

<P>
<PRE>
@$@&lt;ugly duckling@&gt;@L@L+=@{eg@}
@$@&lt;ugly duckling@&gt;@L+=@{sig@}
@$@&lt;ugly duckling@&gt;@L@L+=@{g@}
@$@&lt;ugly duckling@&gt;@L+=@{net@}
</PRE>

<P>Here, two macros having the same
name (<SAMP>ugly duckling</SAMP>) are defined at library
levels one and two. Each of these macros is defined in
two additive parts. The first macro is at level two
and has the value <SAMP>egg</SAMP>. The second macro is
at level one and has the value <SAMP>signet</SAMP>.
If this macro name were referenced by another macro, it
would be expanded to <SAMP>signet</SAMP>, as this is the
value of the definition with the lowest library level.

<P><BR><FONT SIZE="4"><STRONG>Uses Of Library Macros</STRONG></FONT><BR>

<P>When using FunnelWeb as a macro preprocessor (e.g. for
the generation of HTML webs), it's convenient to be able to
define include files that contain large numbers of commonly
used macro definitions. However, sometimes, some macros must
be redefined. By tagging such macros in the include file
using <SAMP>@L</SAMP>, such redefinition is made possible.


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