(VAR w x y z )
(RULES 
        op(i(x), op(y, z)) -> op(f(x, i(x)), op(i(i(y)), z))
        op(i(x), op(y, op(z, w))) -> op(f(x, i(x)), op(z, op(y, w)))
        i(x) -> x
        f(x, y) -> x
        
)