Significant chars in IDs

[VLD/VMB]

C is supposed to support AT LEAST 8 significant characters in
"internal" names and AT LEAST 6, possibly monocase, significant
characters in "external" names.  On the PDP-11, internal names
sometimes are restricted to 7 characters instead of 8 (this is
a bug).

X3J11 specifies AT LEAST 31 signifcant characters in internal
identifiers and macro names and the same restrictions as above
on external identifiers.

There are compilers supporting longer significance in identifiers,
but for portability you must not rely on more than the minimum.