Sorry to bother you with this as I'm sure the answer should be trivial; the first part of the following doesn't work as expected: f(l,m):=A[l,m]+B[l,m]+A[l+1,m]; ev(f(l,m), A[l,m]=0); A[l,m] still appears in the result. However, f(l,m); ev(%, A[l,m]=0 ); works as expected. What am I missing? David