Simplify the Expr.

def simplify(self):
        """Simplify the Expr."""
        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])