added:
smaller?(product(f(x),x),product(f(y),y))
smaller?(product(f(y),y),product(f(x),x))
f:=operator 'f
-- y is replaced with x? product(f(x),x) = product(f(y), y)
test %
smaller?(product(f(x),x), product(f(y), y))
smaller?(product(f(y),y), product(f(x), x))