Information about Compositions
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The functions in this section are actual functions; they compose their
arguments according to the current language and other display modes,
then return a certain measurement of the composition as an integer.

The `cwidth' function measures the width, in characters, of a
composition.  For example, `cwidth(a + b)' is 5, and `cwidth(a / b)'
is 5 in normal mode, 1 in Big mode, and 11 in TeX mode (for `{a \over
b}').  The argument may involve the composition functions described in
this section.

The `cheight' function measures the height of a composition.  This is
the total number of lines in the argument's printed form.

The functions `cascent' and `cdescent' measure the amount of the
height that is above (and including) the baseline, or below the
baseline, respectively.  Thus `cascent(X) + cdescent(X)' always equals
`cheight(X)'.  For a one-line formula like `a + b', `cascent' returns
1 and `cdescent' returns 0.  For `a / b' in Big mode, `cascent'
returns 2 and `cdescent' returns 1.  The only formula for which
`cascent' will return zero is `cvspace(0)' or equivalents.