Gapplet

You input a polynomial f Z[x,y]. We then view it as the recipee for a binary operation on Z, mapping (a,b) Z×Z to f(a,b) Z, and we analyse whether the operation is associative.

Input a polynomial f in x, y with integral coefficients, 
e.g., x*y+x+y.
Test whether the corresponding binary map is associative
 
We compute f(x,f(y,z)) and f(f(x,y),z), and compare the two. By arguments as in Lagrange interpolation, equality of these two polynomials in x, y, z is equivalent to associativity.