insertLeft method
Insert new node to the left
Implementation
ZipperLocation<ZR, ZI, ZS> insertLeft(ZR r) {
if (path is TopPath) {
throw Exception("insert of top");
}
final p = path as NodePath<ZS, ZR>;
return update(
node: node,
path: NodePath(
left: List.unmodifiable([r, ...p.left]),
parentPath: p.parentPath,
parentNode: p.parentNode,
right: p.right,
),
);
}