code: @Override
      public double membership(double x) {
          double result = 0;
          List<InputVariable> inputVariables = engine.getInputVariables();
          final int numberOfVariables = inputVariables.size();
          final int numberOfCoefficients = coefficients.size();
          for (int i = 0; i < numberOfVariables; ++i) {
              if (i < numberOfCoefficients) {
                  result += coefficients.get(i) * inputVariables.get(i).getValue();
              }
          }
          if (numberOfCoefficients > numberOfVariables) {
              result += coefficients.get(numberOfCoefficients - 1);
          }
          return result;
      }
docs: Computes the linear function `f(x)=\sum_i c_iv_i +k`, where `v_i`
  is the value of the input variable `i` registered in the
  Linear::getEngine()
  
  @param x is not utilized
  @return `\sum_i c_ix_i +k`