risr
                                       /    \
                                   risr  is  risr
                                  /    \    /    \
                              risr  is  risr  is  risr
                             /    \    /    \    /    \
                         risr  is  risr  is  risr  is  risr
                        /    \    /    \    /    \    /    \
                    risr  is  risr  is  risr  is  risr  is  risr
                   /    \    /    \    /    \    /    \    /    \
               risr  is  risr  is  risr  is  risr  is  risr  is  risr
              /    \    /    \    /    \    /    \    /    \    /    \
          risr  is  risr  is  risr  is  risr  is  risr  is  risr  is  risr
         /    \    /    \    /    \    /    \    /    \    /    \    /    \
     risr  is  risr  is  risr  is  risr  is  risr  is  risr  is  risr  is  risr