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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def extrusion(target, throat_perimeter='throat.perimeter', throat_length='throat.length'):
 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'. """

    P = target[throat_perimeter]
    L = target[throat_length]
    value = P*L
    return value