* add.f - compensated addition by the Kahan algorithm
      SUBROUTINE ADD (X, N, R)
       IMPLICIT NONE
       INTEGER N
       REAL X, R
       DIMENSION X(N)

       INTEGER K
       DOUBLE PRECISION S, C, Y, T

       S = 0.
       C = 0.
       DO K = 1, N
         Y = X(K) - C
         T = S + Y
         C = (T - S) - Y
         S = T
       END DO
       R = SNGL(S)
       RETURN
      END  ! of ADD
      

* same thing in double precision      
      SUBROUTINE DADD (X, N, S)
       IMPLICIT NONE
       INTEGER N
       DOUBLE PRECISION X, S
       DIMENSION X(N)
       
       INTEGER K
       DOUBLE PRECISION C, Y, T
       
       S = 0.
       C = 0.
       DO K = 1, N
         Y = X(K) - C
         T = S + Y
         C = (T - S) - Y
         S = T
       END DO
       RETURN
      END  ! of DADD
*+++++++++++++++++++++++++ End of file add.f +++++++++++++++++++++++++++