Section 14,91: function_subprogram

 function subprogram

 A series of statements that perform some operation and return a
 single value (through the function name) to the calling program
 unit.  A function subprogram begins with a FUNCTION (or optional
 OPTIONS) statement and is invoked by a function reference in a main
 program unit or a subprogram unit.  See also subroutine_subprogram
 and statement_function.