### `++upas`

    ++  upas                                                ::  tree change (%d)
              $&  [p=upas q=upas]                           ::  cell
              $%  [%0 p=axis]                               ::  copy old
                  [%1 p=*]                                  ::  insert new
                  [%2 p=axis q=udon]                        ::  mutate!
              ==                                            ::

See `%clay` doc