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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_constant(expression):
 """Check if the given expression is constant, i.e. it does not contain Wildcards."""

    if isinstance(expression, Wildcard):
        return False
    if isinstance(expression, Expression):
        return expression.is_constant
    if isinstance(expression, Operation):
        return all(is_constant(o) for o in op_iter(expression))
    return True