code: def vector(x, y=None, z=0.0):
      
      if y is None:  # assume x is already [x,y,z]
          return np.array(x, dtype=np.float64)
      return np.array([x, y, z], dtype=np.float64)
docs: Return a 3D numpy array representing a vector (of type `numpy.float64`).
  
      If `y` is ``None``, assume input is already in the form `[x,y,z]`.