docs: def R_value_to_k(R_value, SI=True):
      r'''Returns the thermal conductivity of a substance given its R-value,
      which can be in either SI units of m^2 K/(W*inch) or the Imperial units
      of ft^2 deg F*h/(BTU*inch).
  
      Parameters
      ----------
      R_value : float
          R-value of a substance [m^2 K/(W*inch) or ft^2 deg F*h/(BTU*inch)]
      SI : bool, optional
          Whether to use the SI conversion or not
  
      Returns
      -------
      k : float
          Thermal conductivity of a substance [W/m/K]
  
      Notes
      -----
      If given input is SI, it is divided by 0.0254 (multiplied by 39.37) and
      then inversed. Otherwise, it is multiplied by 6.93347 and then inversed.
  
      Examples
      --------
      >>> R_value_to_k(0.12), R_value_to_k(0.71, SI=False)
      (0.2116666666666667, 0.20313787163983468)
  
      >>> R_value_to_k(1., SI=False)/R_value_to_k(1.)
      5.678263341113488
  
      References
      ----------
      .. [1] Gesellschaft, V. D. I., ed. VDI Heat Atlas. 2nd edition.
         Berlin; New York:: Springer, 2010.
      '''
      if SI:
          r = R_value/inch
      else:
          r = R_value*foot**2*degree_Fahrenheit*hour/Btu/inch
      return thermal_resistivity_to_k(r)