IronRuby.Builtins.RegexpTransformer.CharacterSet.Union C# (CSharp) Method

            internal CharacterSet/*!*/ Union(CharacterSet/*!*/ set) {
                if (IsEmpty) {
                    return set;
                } else if (set.IsEmpty) {
                    return this;
                }

                if (_negated) {
                    if (set._negated) {
                        // ^A or ^B == ^(A and B)
                        return Complement().Intersect(set.Complement()).Complement();
                    } else {
                        // ^A or B == ^(A \ B)
                        return Complement().Subtract(set).Complement();
                    }
                } else if (set._negated) {
                    // A or ^B == ^(B \ A)
                    return set.Complement().Subtract(this).Complement();
                }

                // (a \ B) or (c \ D) == (a or c) \ ((D \ a) or (B \ c) or (B and D))
                //
                // Proof: 
                // (a \ B) or (c \ D) == 
                // (a and ^B) or (c and ^D) == 
                // (a or c) and (a or ^D) and (^B or c) and (^B or ^D) ==
                // (a or c) \ (^(a or ^D) or ^(^B or c) or ^(^B or ^D)) ==
                // (a or c) \ ((D \ a) or (B \ c) or (B and D))                QED
                return new CharacterSet(_include + set._include,
                    set._exclude.Subtract(GetIncludedSet()).
                        Union(this._exclude.Subtract(set.GetIncludedSet())).
                        Union(this._exclude.Intersect(set._exclude))
                );
            }