The PROGRAM statement can only appear at the start of the main
program unit. Its only function is to indicate what type of program unit
it is and to give it symbolic name. Although this name cannot be used
anywhere else in the program, it may be used by the Fortran system to
identify error messages etc. The general form is simply:
PROGRAM name
Where name is a symbolic name. This name is global in scope and
may not be used elsewhere in the main program nor as a global name in any
other program unit. For compatibility with Fortran66 the PROGRAM
statement is optional. This can have unexpected effects: if you forget
use a SUBROUTINE or FUNCTION statement at the start of a
procedure the compiler will assume it to be a (nameless) main program
unit. Since this will normally result in two main program units, the
linker is likely to detect the mistake.