insertDown method
Insert down
Implementation
ZipperLocation<ZR, ZI, ZS> insertDown(ZR d) {
if (!sectionP(node)) {
throw Exception("down of item");
}
final t = node as ZS;
return update(
node: d,
path: NodePath<ZS, ZR>(
left: List<ZR>.unmodifiable([]),
parentPath: path,
parentNode: node,
right: getChildren(t),
),
);
}