code: @Override
      public double membership(double x) {
          if (Double.isNaN(x)) {
              return Double.NaN;
          }
          if (Op.isGE(x, start) && Op.isLE(x, end)) {
              return height * 1.0;
          }
          return height * 0.0;
      }
docs: Computes the membership function evaluated at `x`
  
  @param x
  @return `\begin{cases} 1h & \mbox{if $x \in [s, e]$} \cr 0h &
  \mbox{otherwise} \end{cases}`
  
  where `h` is the height of the Term,
  `s` is the start of the Rectangle,
  `e` is the end of the Rectangle.