// remarkably it is even faster than hoisting the bounds check
// out of the loop, by doing something like
//   _, _ = x[len(z)-1], y[len(z)-1]
// There are other ways to hoist the bounds check out of the loop,
// but the compiler's BCE isn't powerful enough for them (yet?).
// See the discussion in CL 164966.

// ----------------------------------------------------------------------------
// Elementary operations on words
//