private void StepDel(ISet<Production> productions) {
var nullableProbabilities = GrammarHelpers.GetNullable(productions);
var newRules = new List<Production>();
foreach (var production in productions) {
var toAdd = Nullate(production, nullableProbabilities);
RemoveExtraneousNulls(toAdd);
newRules.AddRange(toAdd);
}
productions.Clear();
productions.UnionWith(newRules);
}