extensions/references-view/src/references/model.ts/ReferencesModel/_move
class ReferencesModel:	private _move(item: FileItem | ReferenceItem, fwd: boolean): ReferenceItem | void {

		const delta = fwd ? +1 : -1;

		const _move = (item: FileItem): FileItem => {
			const idx = (this.items.indexOf(item) + delta + this.items.length) % this.items.length;
			return this.items[idx];
		};

		if (item instanceof FileItem) {
			if (fwd) {
				return _move(item).references[0];
			} else {
				return tail(_move(item).references);
			}
		}

		if (item instanceof ReferenceItem) {
			const idx = item.file.references.indexOf(item) + delta;
			if (idx < 0) {
				return tail(_move(item.file).references);
			} else if (idx >= item.file.references.length) {
				return _move(item.file).references[0];
			} else {
				return item.file.references[idx];
			}
		}
	}