  An asymmetric double-well potential is considered, assuming that the minima
of the wells are quadratic with a frequency $\omega$ and the difference of the
minima is close to a multiple of $\hbar \omega$. A WKB wave function is
constructed on both sides of the local maximum between the wells, by matching
the WKB function to the exact wave functions near the classical turning points.
The continuities of the wave function and its first derivative at the local
maximum then give the energy-level splitting formula, which not only reproduces
the instanton result for a symmetric potential, but also elucidates the
appearance of resonances of tunneling in the asymmetric potential.
