':param function: ``Expression``, for the function expression
:param argument: ``Expression``, for the argument'

def __init__(self, function, argument):

    assert isinstance(function, Expression), (u'%s    is    not    an    Expression' % function)
    assert isinstance(argument, Expression), (u'%s    is    not    an    Expression' % argument)
    self.function = function
    self.argument = argument
