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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 get_variables(expression, variables=None):
 """Returns the set of variable names in the given expression."""

    if variables is None:
        variables = set()
    if hasattr(expression, 'variable_name') and expression.variable_name is not None:
        variables.add(expression.variable_name)
    if isinstance(expression, Operation):
        for operand in op_iter(expression):
            get_variables(operand, variables)
    return variables