buildPath

assert(DirPath("/etc").buildPath(FileName("foo")) == FilePath("/etc/foo"));
assert(DirPath("/etc").buildPath(FilePath("foo")) == FilePath("/etc/foo"));
assert(DirPath("/usr").buildPath(DirName("bin")) == DirPath("/usr/bin"));
assert(DirPath("/usr").buildPath(DirPath("bin")) == DirPath("/usr/bin"));
assert(DirPath("/usr").buildPath(DirPath("/bin")) == DirPath("/bin"));