code: @Override
      public double membership(double x) {
          if (Double.isNaN(x)) {
              return Double.NaN;
          }
          return height * 1.0 / (1.0 + Math.exp(-slope * (x - inflection)));
      }
docs: Computes the membership function evaluated at `x`
  
  @param x
  @return ` h / (1 + \exp(-s(x-i)))`
  
  where `h` is the height of the Term,
  `s` is the slope of the Sigmoid,
  `i` is the inflection of the Sigmoid