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that need this kind of ability include graphical user interfaces, image
processing libraries, mixed-precision numerical computation systems, and
most types of simulations.</FONT></FONT>

<P><FONT SIZE=-1><FONT FACE="Times">It's possible to build "hand-crafted"
multiply dispatched methods that look at the types of each of their arguments
and react accordingly. For example, a normal (<I>singly-dispatched</I>)
method could use </FONT><FONT FACE="Courier">ref</FONT><FONT FACE="Times">
or </FONT><FONT FACE="Courier">isa</FONT><FONT FACE="Times"> to determine
the types of its other arguments and then select the correct behaviour
in a cascaded </FONT><FONT FACE="Courier">if</FONT><FONT FACE="Times">
statement. Alternatively, it's possible to use hashes of hashes to set
up a multidimensional table of subroutine references, then use the class
names of the arguments (again found with </FONT><FONT FACE="Courier">ref</FONT><FONT FACE="Times">)
to index into it. Both these approaches are described in detail in <A HREF="#references">[1,2]</A>.</FONT></FONT>

<P><FONT FACE="Times"><FONT SIZE=-1>The problem is that such hand-crafted
mechanisms are complicated to construct and even harder to debug. And because
every hand-built method is structurally similar, they're also tedious to
code and maintain. Life would be very much easier if it were possible to
define a set of identically named methods with distinct parameter lists,