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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 cylinder(target, throat_diameter='throat.diameter'):
 r""" Calculate throat cross-sectional area for a cylindrical 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 of the throat diameter values """

    diams = target[throat_diameter]
    value = _pi/4*(diams)**2
    return value