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4.4.1.1 '\charsubdef': Character substitutions
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The most important primitive MLTeX adds is '\charsubdef', used in a way
reminiscent of '\chardef':
     \charsubdef COMPOSITE [=] ACCENT BASE

   Each of COMPOSITE, ACCENT, and BASE are font glyph numbers, expressed
in the usual TeX syntax: `\e symbolically, '145 for octal, "65 for hex,
101 for decimal.

   MLTeX's '\charsubdef' declares how to construct an accented character
glyph (not necessarily existing in the current font) using two character
glyphs (that do exist).  Thus it defines whether a character glyph code,
either typed as a single character or using the '\char' primitive, will
be mapped to a font glyph or to an '\accent' glyph construction.

   For example, if you assume glyph code 138 (decimal) for an
e-circumflex and you are using the Computer Modern fonts, which have the
circumflex accent in position 18 and lowercase 'e' in the usual ASCII
position 101 decimal, you would use '\charsubdef' as follows:

     \charsubdef 138 = 18 101

   For the plain TeX format to make use of this substitution, you have
to redefine the circumflex accent macro '\^' in such a way that if its
argument is character 'e' the expansion '\char138 ' is used instead of
'\accent18 e'.  Similar '\charsubdef' declaration and macro
redefinitions have to be done for all other accented characters.

   To disable a previous '\charsubdef C', redefine C as a pair of zeros.
For example:
     \charsubdef '321 = 0 0  % disable N tilde
(Octal '321 is the ISO Latin-1 value for the Spanish N tilde.)

   '\charsubdef' commands should only be given once.  Although in
principle you can use '\charsubdef' at any time, the result is
unspecified.  If '\charsubdef' declarations are changed, usually either
incorrect character dimensions will be used or MLTeX will output missing
character warnings.  (The substitution of a '\charsubdef' is used by TeX
when appending the character node to the current horizontal list, to
compute the width of a horizontal box when the box gets packed, and when
building the '\accent' construction at '\shipout'-time.  In summary, the
substitution is accessed often, so changing it is not desirable, nor
generally useful.)