Begins a main program.  The PROGRAM statement is optional; when
  used, it can only be preceded by comment lines or an OPTIONS
  statement.  Statement format:

     PROGRAM nam

     nam   Is a symbolic name for the program.  The name must
           be unique among all global names in the program.

  If no PROGRAM statement begins the program, the program name
  defaults to filename$MAIN, where filename is the name of the file
  containing the program.

  The main program cannot contain the following attributes:  INTENT,
  OPTIONAL, PRIVATE, or PUBLIC.

  A main program can contain an internal subprogram (defines an
  internal procedure).  It must be preceded by a CONTAINS statement.