code: @Override
      public double membership(double x) {
          if (Double.isNaN(x)) {
              return Double.NaN;
          }
          double mu = 0.0;
          for (Activated term : this.terms) {
              mu = this.aggregation.compute(mu, term.membership(x));
          }
          return mu;
      }
docs: Aggregates the membership function values of `x` utilizing the
  aggregation operator
  
  @param x is a value
  @return `\sum_i{\mu_i(x)}, i \in \mbox{terms}`