varsMonomials :: [Monomial] -> [Var]
varsMonomials ((Monomial _ vs):ms) = vs `union` (varsMonomials ms)
varsMonomials :: [Monomial] -> [Var]
varsMonomials ((Monomial _ vs):ms) = vs `union` (varsMonomials ms)
varsMonomials ((Monomial _ vs):ms) = vs `union` (varsMonomials ms)