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.
