code: function SumFactor(sum_variable, terms_variables, coeffs) {
        if (len(terms_variables) !== len(coeffs)) {
          throw new Error("assert error");
        }
        this.sum = sum_variable;
        this.terms = terms_variables;
        this.coeffs = coeffs;
        SumFactor.__super__.constructor.call(this, [sum_variable].concat(terms_variables));
      }
docs: /* A factor that connects a sum variable with 1 or more terms,
  which are summed after being multiplied by fixed (real)
  coefficients.