PROGRAM main
    IMPLICIT NONE
    REAL :: X

    CALL diff (R=4.56, Q=1.23, P=X)
    PRINT *, X
    CALL diff (Q=-7.89, P=X, R=3.42)
    PRINT *, X
    CALL diff (R=-0.98, P=X, Q=4.29)
    PRINT *, X

CONTAINS

    SUBROUTINE diff (P, Q, R)
        IMPLICIT NONE
        REAL, INTENT(OUT) :: P
        REAL, INTENT(IN) :: Q, R
        P = Q-R
    END SUBROUTINE diff

END PROGRAM main