! 4d space-time
FUNCTION space_time( x, y, z, t ) RESULT( ss )

  IMPLICIT NONE
  REAL, INTENT(IN) :: x, y, z
  REAL, INTENT(IN), OPTIONAL :: t
  REAL :: ss

  ss = x * x + y * y + z * z
  IF ( PRESENT( t ) ) ss = ss - t * t

END FUNCTION space_time

! main
PROGRAM main

  IMPLICIT NONE
  REAL :: a, b, c, t

  INTERFACE
    FUNCTION space_time( x, y, z, t ) RESULT( ss )
      REAL, INTENT(IN) :: x, y, z
      REAL, INTENT(IN), OPTIONAL :: t
      REAL :: ss
    END FUNCTION
  END INTERFACE

  a = 1.0E0
  b = 2.0E0
  c = 1.0E0
  PRINT '(F7.4)', space_time( x = a, y = b, z = c )  ! t 引数は，なし
  t = 1.5E0
  PRINT '(F7.4)', space_time( a, b, c, t )

END PROGRAM main