r2 = u2>>n | u1<<(64-n) | u1>>(n-64)
	r1 = u1 >> n
	return
}

// shrcompress compresses the bottom n+1 bits of the two-word
// value into a single bit. the result is equal to the value
// shifted to the right by n, except the result's 0th bit is
// set to the bitwise OR of the bottom n+1 bits.
func shrcompress(u1, u2 uint64, n uint) (r1, r2 uint64) {
	// TODO: Performance here is really sensitive to the