def __init__(self, nominal_value, error = 0):
        
        """
        'nominal_value' is the nominal value of the semi-formal variable.
        'error' is the error on the value.
        """

        # We initialize the value in the same way as users of
        # instances who would set it: users use the 'nominal_value' attribute:
        self.nominal_value = nominal_value  # Defines the (constant) expression
        self.error = error
        # Expression 'x' depends on 'x':
        self._variables = set([self])

        'nominal_value' is the nominal value of the semi-formal variable.
        'error' is the error on the value.
        
recipes/Python/576721_Calculations_error_propagatisemiformal/recipe-576721.py