'Returns the arbitary expression which is to have the Wavefunction
substituted into it
Examples
>>> from sympy.physics.quantum.operator import DifferentialOperator
>>> from sympy import Function, Symbol, Derivative
>>> x = Symbol(\'x\')
>>> f = Function(\'f\')
>>> d = DifferentialOperator(Derivative(f(x), x), f(x))
>>> d.expr
Derivative(f(x), x)
>>> y = Symbol(\'y\')
>>> d = DifferentialOperator(Derivative(f(x, y), x) +
...                          Derivative(f(x, y), y), f(x, y))
>>> d.expr
Derivative(f(x, y), x) + Derivative(f(x, y), y)'

@property
def expr(self):

    return self.args[0]
