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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 is_anonymous(expression):
 """Returns True iff the expression does not contain any variables."""

    if hasattr(expression, 'variable_name') and expression.variable_name:
        return False
    if isinstance(expression, Operation):
        return all(is_anonymous(o) for o in op_iter(expression))
    return True