SUBROUTINE CPROD(N1,N2,LIST1,LIST2,ZZ,LDZ,XY,LDXY)
C  This routine extracts the sums of cross products of a subset of 
C  the variables in a sums-of-squares-and-cross-products matrix.
C  It is useful for reordering variables.
C  Inuts:
C  N1      Number of rows in output sums-of-cross-products matrix
C  N2      Number of columns in output sums-of-cross-products matrix
C  LIST1   N1x1 vector of ZZ entries corresponding to rows of XY
C  LIST1   N2x1 vector of ZZ entries corresponding to columns of XY
C  ZZ      Sums-of-squares-and-cross-products matrix
C  LDZ     Leading dimension of Z
C  LDXY    Leading dimension of XY
C  Output:
C  XY      N1xN2 sums-of-cross-products matrix
      IMPLICIT REAL*8 (A-H,O-Z)
      PARAMETER(LD=100)
      COMMON/SCRA/V1(LD),V2(LD),A1(LD,LD),A2(LD,LD)
      DIMENSION LIST1(N1),LIST2(N2),ZZ(LDZ,LDZ),XY(LDXY,LDXY)
      DO 10 I1=1,N1
      J1=LIST1(I1)
      DO 10 I2=1,N2
      J2=LIST2(I2)
   10 A1(I1,I2)=ZZ(J1,J2)
      DO 20 I1=1,N1
      DO 20 I2=1,N2
   20 XY(I1,I2)=A1(I1,I2)
      RETURN
      END