      FUNCTION RNDERR(X, A, Y, B)
C       THIS FUNCTION COMPUTES THE ROUNDING ERROR COMMITTED WHEN THE
C       SUM X+A IS FORMED.  IN THE CALLING PROGRAM, Y MUST BE THE SAME
C       AS X AND B MUST BE THE SAME AS A.  THEY ARE DECLARED AS
C       DISTINCT VARIABLES IN THIS FUNCTION, AND THE INTERMEDIATE
C       VARIABLES S AND T ARE PUT INTO COMMON, IN ORDER TO DEFEND
C       AGAINST THE WELL-MEANING ACTIONS OF SOME OFFICIOUS OPTIMIZING
C       FORTRAN COMPILERS.
      COMMON /CUBATB/ S, T
      S = X + A
      T = S - Y
      RNDERR = T - B
      RETURN
      END
