function a=cellfac(x1,x2,y1,y2,the,phi)
k=2*pi;
a1=cos(k*x2.*sin(the).*cos(phi))-cos(k*x1.*sin(the).*cos(phi)); 
a2=sin(k*x2.*sin(the).*cos(phi))-sin(k*x1.*sin(the).*cos(phi));
a3=cos(k*y1.*sin(the).*sin(phi))-cos(k*y1.*sin(the).*sin(phi));
a4=sin(k*y2.*sin(the).*sin(phi))-sin(k*y1.*sin(the).*sin(phi));
a=(a1+i*a2).*(a3+i*a4);