src/vs/base/common/skipList.ts/SkipList/_randomLevel
class SkipList:	private static _randomLevel(list: SkipList<any, any>, p: number = 0.5): number {
		let lvl = 1;
		while (Math.random() < p && lvl < list._maxLevel) {
			lvl += 1;
		}
		return lvl;
	}