docs: def V_hollow_cylinder(Di, Do, L):
      r'''Returns the volume of a hollow cylinder.
  
      .. math::
          V = \frac{\pi D_o^2}{4}L - L\frac{\pi D_i^2}{4}
  
      Parameters
      ----------
      Di : float
          Diameter of the hollow in the cylinder, [m]
      Do : float
          Diameter of the exterior of the cylinder, [m]
      L : float
          Length of the cylinder, [m]
  
      Returns
      -------
      V : float
          Volume [m^3]
  
      Examples
      --------
      >>> V_hollow_cylinder(0.005, 0.01, 0.1)
      5.890486225480862e-06
      '''
      V = pi*Do**2/4*L - pi*Di**2/4*L
      return V