|   X---+---+---@   |   |
|   Y---+---O---@---O   |
|   H   |   |   |   |   |
|   <---+---+--->   |   |
M   |   |   |   |   |   |
@---X   |   |   |   |   |
@---+---+---+---+---X   |
@---+---+---O---+---X   |