PRESENT (opt-argument)

  Class:  Inquiry function - Generic

  Returns whether or not an optional dummy argument is present (has
  an associated actual argument).

  Example:

  Consider the following:

  SUBROUTINE CHECK (X, Y)
    REAL X, Z
    REAL, OPTIONAL :: Y
    ...
    IF (PRESENT (Y)) THEN
      Z = Y
    ELSE
       Z = X * 2
    END IF
  END
  ...
  CALL CHECK (15.0, 12.0)      ! Causes B to be set to 12.0
  CALL CHECK (15.0)            ! Causes B to be set to 30.0