OPTIONAL

 Permits dummy arguments to be omitted in a procedure reference.

 The OPTIONAL attribute can be specified in a type declaration
 statement or an OPTIONAL statement, and takes one of the following
 forms:

 Type Declaration Statement:

  type, [att-ls,] OPTIONAL [,att-ls] :: d-arg [,d-arg]...

 Statement:

  OPTIONAL [::] d-arg [,d-arg]...

    type      Is a data type specifier.

    att-ls    Is an optional list of attribute specifiers.

    d-arg     Is the name of a dummy argument.

 The OPTIONAL attribute can only appear in the scoping unit of a
 subprogram or an interface body, and can only be specified for
 dummy arguments.

 A dummy argument is "present" if it associated with an actual
 argument.  A dummy argument that is not optional must be present.
 You can use the PRESENT intrinsic function to determine whether an
 optional dummy argument is associated with an actual argument.

 To call a procedure that has an optional argument, you must use an
 explicit interface.

 The OPTIONAL attribute is compatible with the DIMENSION, EXTERNAL,
 INTENT, POINTER, TARGET, and VOLATILE attributes.

 EXAMPLES:

 The following example shows a type declaration statement specifying
 the OPTIONAL attribute:

   SUBROUTINE TEST(A)
   REAL, OPTIONAL, DIMENSION(-10:2) :: A
   END SUBROUTINE

 The following is an example of the OPTIONAL statement:

        SUBROUTINE TEST(A, B, L, X)
        OPTIONAL :: B
        INTEGER A, B, L, X

        IF (PRESENT(B)) THEN        ! Printing of B is conditional
           PRINT *, A, B, L, X      !   on its presence
        ELSE
           PRINT *, A, L, X
        ENDIF
        END SUBROUTINE

        INTERFACE
           SUBROUTINE TEST(ONE, TWO, THREE, FOUR)
            INTEGER ONE, TWO, THREE, FOUR
            OPTIONAL :: TWO
          END SUBROUTINE
        END INTERFACE

        INTEGER I, J, K, L

        I = 1
        J = 2
        K = 3
        L = 4

        CALL TEST(I, J, K, L)            ! Prints:  1  2  3  4
        CALL TEST(I, THREE=K, FOUR=L)    ! Prints:  1  3  4
        END

 Note that in the second call to subroutine TEST, the second
 positional (optional) argument is omitted.  In this case, all
 following arguments must be keyword arguments.