(Corresponds to Section 3.4 of ANSI X3.9-1978 FORTRAN 77.)
A statement separated from its predecessor via a semicolon may be
labeled as follows:
- The semicolon is followed by the label for the statement,
which in turn follows the label.
- The label must be no more than five digits in length.
- The first digit of the label for the statement is not
the first non-space character on a line.
Otherwise, that character is treated as a continuation
A statement may have only one label defined for it.