Finally, we would like to be able to use TEX's built-in booleans
as well as our own. For example, we would like a predicate
Lessthan such that:
#math116#
| Lessthan~i~j |
= |
#tex2html_wrap_indisplay2333##tex2html_wrap_indisplay2334# |
|
The Error condition happens if we try applying Lessthan to something
that isn't a number --- #math117#Lessthan~True~False is Error36
1.
This is fine as a mathematical definition, but how will
we implement it? If we assume we have a macro 22,
which converts TEX if-statements into booleans, we could just
define:
So the question is just how to define 23.
Unfortunately, the ``obvious'' code does not work:
verbatim69#
For example, 24 doesn't expand out to
25. Instead, it expands as:
verbatim70#
Another common TEXnique is to use a macro 26 to
be the expansion text:
verbatim71#
However, this uses TEX's stomach to do the 27, and we are
trying to do this using only the mouth. One (slightly tricky) solution
is to use pattern-matching to gobble up the offending 28 and/or
29.
So if the TEX if-statement is true, 30 gobbles
up the false-text, otherwise 31 gobbles up the
true-text. For example,
verbatim72#
Phew. And so we have booleans.