Add two swift dictionaries together into a new optional dictionary, result is nil if there were conflicting keys

Dictionary.swift

func +<Key, Value>(lhs: [Key: Value], rhs: [Key: Value]) -> [Key: Value]? {
	var result = lhs
	for (key, value) in rhs {
		if contains(lhs.keys, key) {
			return nil
		}
		result[key] = value
	}
	return result
}