code: func (rk *rowKey) pathPrefixUpperBound() (v string) {
  	withRowKeyBuffers(func(rkb *rowKeyBuffers) {
  		rkb.appendPathPrefix(rk.pathHash)
  		rkb.appendBytes([]byte("~~"))
  		v = rkb.value()
  	})
  	return
  }
docs: // pathPrefixUpperBound returns the path prefix that is higher than any path
  // allowed in the row key space.
  //
  // This is accomplished by appending a "~" character to the path prefix,
  // creating something like this:
  //
  //     prefix~~
  //
  // The "prefix~" is shared with all keys in "rk", but the extra "~" is larger
  // than any hex-encoded row index, so this key will always be larger.