func (g *generation) walk(f func(rev revision) bool) int {
	l := len(g.revs)
	for i := range g.revs {
		ok := f(g.revs[l-i-1])
		if !ok {
			return l - i - 1
		}
	}
	return -1
}
func (g *generation) walk(f func(rev revision) bool) int {
	l := len(g.revs)
	for i := range g.revs {
		ok := f(g.revs[l-i-1])
		if !ok {
			return l - i - 1
		}
	}
	return -1
}
// walk walks through the revisions in the generation in descending order.
// It passes the revision to the given function.
// walk returns until: 1. it finishes walking all pairs 2. the function returns false.
// walk returns the position at where it stopped. If it stopped after
// finishing walking, -1 will be returned.