def mutable_block(self, x: int, y: int) -> Block:
        """Returns the block at (x, y) so it can be edited."""
        if x < 0 or y < 0:
            raise IndexError('x < 0 or y < 0')
        return self._blocks[(x, y)]
Returns the block at (x, y) so it can be edited.
cirq-core/cirq/circuits/_block_diagram_drawer.py