**==isum.f  
C++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
C
C*************************
C**       ISUM          **
C*************************
C
C
      INTEGER  FUNCTION  ISUM (IX,SKIP,N)
C
C Purpose: To add up n elements of the vector ix, skipping 'skip' elements
C          between each addition
C
      INTEGER  SKIP, N
      INTEGER  IX(SKIP,N)
C
C====== LOCALS:
C
      INTEGER  I
C
C------------------------------------------------------------------------------
C
      ISUM = 0
      DO 20 I = 1 , N
         ISUM = ISUM + IX(1,I)
 20   CONTINUE
      RETURN
      END
