goUp method
Move to parent
Implementation
ZipperLocation<ZR, ZI, ZS> goUp() {
if (path is TopPath) {
throw Exception("up of top");
}
NodePath<ZS, ZR> p = path as NodePath<ZS, ZR>;
List<ZR> cs = List.unmodifiable([...p.left.reversed, node, ...p.right]);
return update(node: makeSection(p.parentNode, cs), path: p.parentPath);
}