def __init__(self, func, variables):
        """
        Node for a semi-formal expression (Semi_formal_expr).
        
        For the meaning of object attributes see the documentation for
        Semi_formal_expr.
        """
        self._evaluate = func  # Way of evaluating the function
        self._variables = variables

        Node for a semi-formal expression (Semi_formal_expr).
        
        For the meaning of object attributes see the documentation for
        Semi_formal_expr.
        
recipes/Python/576721_Calculations_error_propagatisemiformal/recipe-576721.py