CFGLib.GrammarHelpers.GetNullableNonterminals C# (CSharp) Method

		private static ISet<Nonterminal> GetNullableNonterminals(ISet<Production> originalProductions) {
			var productions = CloneProductions(originalProductions);
			ISet<Production> newProductions = new HashSet<Production>();
			var nullableNonterminals = new HashSet<Nonterminal>();
			var changed = true;
			while (changed) {
				changed = false;
				foreach (var production in productions) {
					if (production.IsEmpty) {
						nullableNonterminals.Add(production.Lhs);
						changed = true;
						continue;
					}
					if (production.Rhs.OnlyTerminals()) {
						continue;
					}
					for (int i = production.Rhs.Count - 1; i >= 0; i--) {
						var word = production.Rhs[i];
						if (nullableNonterminals.Contains(word)) {
							production.Rhs.RemoveAt(i);
							changed = true;
						}
					}
					newProductions.Add(production);
				}
				var oldProductions = productions;
				productions = newProductions;
				newProductions = oldProductions;
				newProductions.Clear();
			}
			return nullableNonterminals;
		}