Convert to the matrix.

def to_matrix(self, n_qubits=-1):
        """Convert to the matrix."""
        if self.is_identity:
            if n_qubits == -1:
                return self.matrix
            else:
                return reduce(np.kron, [I.matrix for _ in range(n_qubits)])
        if n_qubits == -1:
            n_qubits = _n(self) + 1
        if _n(self) == 0:
            mat = self.matrix
        else:
            mat = reduce(np.kron, [I.matrix for _ in range(_n(self))])
            mat = np.kron(mat, self.matrix)
        if n_qubits > _n(self) + 1:
            mat = reduce(np.kron, [I.matrix for _ in range(n_qubits - _n(self) - 1)], mat)
        return mat