code: def simplify(self):
          
          d = defaultdict(float)
          for term in self.terms:
              term = term.simplify()
              d[term.ops] += term.coeff
          return Expr.from_terms_iter(
              Term.from_ops_iter(k, d[k]) for k in sorted(d, key=repr) if d[k])
docs: Simplify the Expr.