code: def cuboid(target, throat_diameter='throat.diameter',
             throat_length='throat.length'):
      r
      D = target[throat_diameter]
      L = target[throat_length]
      value = 4*D*L
      return value
docs: r"""
      Calculate surface area for a cuboid throat
  
      Parameters
      ----------
      target : OpenPNM Object
          The object which this model is associated with. This controls the
          length of the calculated array, and also provides access to other
          necessary properties.
  
      throat_diameter : string
          Dictionary key to the throat diameter array.  Default is
          'throat.diameter'.
  
      throat_length : string
          Dictionary key to the throat length array.  Default is 'throat.length'.