code: def extrusion(target, throat_perimeter='throat.perimeter',
                throat_length='throat.length'):
      r
      P = target[throat_perimeter]
      L = target[throat_length]
      value = P*L
      return value
docs: r"""
      Calculate surface area for an arbitrary shaped throat give the perimeter
      and length.
  
      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_perimeter : string
          Dictionary key to the throat perimeter array.  Default is
          'throat.perimeter'.
  
      throat_length : string
          Dictionary key to the throat length array.  Default is 'throat.length'.