void bksub(ne,nb,jf,k1,k2,c)
int ne,nb,jf,k1,k2;
float ***c;
{
	int nbf,im,kp,k,j,i;
	float xx;

	nbf=ne-nb;
	im=1;
	for (k=k2;k>=k1;k--) {
		if (k == k1) im=nbf+1;
		kp=k+1;
		for (j=1;j<=nbf;j++) {
			xx=c[j][jf][kp];
			for (i=im;i<=ne;i++)
				c[i][jf][k] -= c[i][j][k]*xx;
		}
	}
	for (k=k1;k<=k2;k++) {
		kp=k+1;
		for (i=1;i<=nb;i++) c[i][1][k]=c[i+nbf][jf][k];
		for (i=1;i<=nbf;i++) c[i+nb][1][k]=c[i][jf][kp];
	}
}