import XOp, PxOp, PxKet
    >>> from sympy.physics.quantum.represent import rep_expectation
    >>> rep_expectation(XOp())
    x_1*DiracDelta(x_1 - x_2)
    >>> rep_expectation(XOp(), basis=PxOp())
    <px_2|*X*|px_1>
    >>> rep_expectation(XOp(), basis=PxKet())
    <px_2|*X*|px_1>

    r3