code: Expression get( int idx ) {
          if( parameters.size() <= idx ) {
              throw new ParameterOutOfBoundsException();
          }
          return parameters.get( idx );
      }
docs: Get the idx parameter from the parameter list.
  
  @param idx
  the index starting with 0
  @return the expression
  @throws ParameterOutOfBoundsException if the parameter with the index does not exists