(Corresponds to Section 2.2 of ANSI X3.9-1978 FORTRAN 77.)
In GNU Fortran, a symbolic name is at least one character long,
and has no arbitrary upper limit on length.
However, names of entities requiring external linkage (such as
external functions, external subroutines, and COMMON areas)
might be restricted to some arbitrary length by the system.
Such a restriction is no more constrained than that of one
through six characters.
Underscores (_) are accepted in symbol names after the first
character (which must be a letter).