code: public static ButtonGroup leftShift(ButtonGroup self, AbstractButton b) {
          self.add(b);
          return self;
      }
docs: Overloads the left shift operator to provide an easy way to add
  buttons to a ButtonGroup.
  
  @param self a ButtonGroup
  @param b    an AbstractButton to be added to the buttonGroup.
  @return same buttonGroup, after the value was added to it.
  @since 1.6.4