**Input:** A qubit in state $|\psi\rangle = \alpha |0\rangle + \beta |1\rangle$.

**Goal:** Change the state of the qubit to $\alpha |1\rangle + \beta |0\rangle$.

**Example:**

If the qubit is in state $|0\rangle$, change its state to $|1\rangle$.

If the qubit is in state $|1\rangle$, change its state to $|0\rangle$.

> Note that this operation is self-adjoint: applying it for a second time
> returns the qubit to the original state.
