!-----  Ex8_6: INTERFACEの例   -----
   PROGRAM main
      INTERFACE
         REAL FUNCTION trace(sqr)
              REAL, INTENT(IN) :: sqr(:,:)
         END FUNCTION
      END INTERFACE   
!
      REAL, ALLOCATABLE :: a(:,:)
      INTEGER :: n, i
      PRINT*, '正方行列の行（または列）の数は？'
      READ*, n
      ALLOCATE( a(n,n) )
      DO i = 1, n
         PRINT '(A, I2, A)', '第', i, '行の要素？'
         READ *, a(i, 1:n)
      END DO
      PRINT '(A, F7.4)', 'Trace = ', trace(a)
   END
!-----   Trace of a Square Matrix
   FUNCTION  trace(x)  RESULT(tr)
      REAL, INTENT(IN) :: x(:,:)
      REAL :: tr
      INTEGER :: n1, n2, i
      n1 = SIZE(x, 1);  n2 = SIZE(x, 2)
      IF( n1 /= n2 ) THEN
          PRINT*,'正方行列ではありません。' ; RETURN
      ELSE
             tr = 0.0
          DO i = 1, n1
             tr = tr + x(i, i)
          END DO
      END IF
   END FUNCTION