docs: def from_spherical(cls, r=1.0, alpha=0.0, delta=0.0):
          """Construct Cartesian vector from spherical coordinates.
  
          alpha and delta must be in radians.
          """
          x = r * math.cos(delta) * math.cos(alpha)
          y = r * math.cos(delta) * math.sin(alpha)
          z = r * math.sin(delta)
          return cls(x=x, y=y, z=z)