src/vs/workbench/contrib/callHierarchy/browser/callHierarchyTree.ts/IdentityProvider/getId
class IdentityProvider:	getId(element: Call): { toString(): string } {
		let res = this.getDirection() + JSON.stringify(element.item.uri) + JSON.stringify(element.item.range);
		if (element.parent) {
			res += this.getId(element.parent);
		}
		return res;
	}