'Returns the variables with which the function in the specified
arbitrary expression is evaluated
Examples
>>> from sympy.physics.quantum.operator import DifferentialOperator
>>> from sympy import Symbol, Function, Derivative
>>> x = Symbol(\'x\')
>>> f = Function(\'f\')
>>> d = DifferentialOperator(1/x*Derivative(f(x), x), f(x))
>>> d.variables
(x,)
>>> y = Symbol(\'y\')
>>> d = DifferentialOperator(Derivative(f(x, y), x) +
...                          Derivative(f(x, y), y), f(x, y))
>>> d.variables
(x, y)'

@property
def variables(self):

    return self.args[(-1)].args
