5.1.1 Programs with Line Numbers
  If you use line numbers in your program, you must follow
  these rules:

  .
        A line number must be a unique integer from 1 to 32767.
        If a VAX BASIC, if a program contains duplicate line
        numbers, the last line with that number replaces the pre-
        vious one. DEC BASIC does not allow programs to have
        duplicate line numbers.
  .
        A line number can contain leading zeros; however, em-
        bedded spaces, tabs, and commas are invalid in line
        numbers.
  .
        There must be a line number on the first line of the
        program.
  .
        If a source file contains subprograms, then each subpro-
        gram must begin on a numbered line.

  Note, that in a multiple-unit program with line num-
  bers, any comments following an END, END SUB, or END
  FUNCTION statement become a part of the previous sub-
  program during compilation unless they begin on a numbered
  line. This is not the case in multiple-unit programs without
  line numbers.

  Although line numbers are not required, you may want to
  use them on every line that can cause a run-time error, de-
  pending on the type of error handling you use. See Chapter
  16 for more information about handling run-time errors.