code: def from_polar(r, theta, phi):
      
      rxy = r * cos(theta)
      return array((rxy * cos(phi), rxy * sin(phi), r * sin(theta)))
docs: Convert ``(r, theta, phi)`` to Cartesian coordinates ``[x y z]``.
  
      ``r`` - vector length
      ``theta`` - angle above (+) or below (-) the xy-plane
      ``phi`` - angle around the z-axis
  
      The meaning and order of the three polar parameters is designed to
      match both ISO 31-11 and the traditional order used by physicists.
      Mathematicians usually define ``theta`` and ``phi`` the other way
      around, and may need to use caution when calling this function.
      See: https://en.wikipedia.org/wiki/Spherical_coordinate_system