goDown method
Move down the tree
Implementation
ZipperLocation<ZR, ZI, ZS> goDown() {
if (!sectionP(node)) {
throw Exception("down of item");
}
final t = node as ZS;
final cs = getChildren(t);
if (cs.isEmpty) {
throw Exception("down of empty");
}
return update(
node: cs.first,
path: NodePath<ZS, ZR>(
left: List<ZR>.unmodifiable([]),
right: List<ZR>.unmodifiable(cs.skip(1)),
parentPath: path,
parentNode: node,
),
);
}