/// Makes a path relative to some base path. That is, base/result==path.
Path relative(const Path &path, const Path &base);
+/// Returns the longest prefix shared by both paths.
+Path common_ancestor(const Path &, const Path &);
+
/** Determines how many levels a path is below another. Returns -1 if path is
not a descendant of parent. */
int descendant_depth(const Path &path, const Path &parent);