code: def to_matrix(self, n_qubits=-1):
          
          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
docs: Convert to the matrix.