Append to array inside a dictionary without copying

DictionaryAppend.swift

/// Appends a value to an array within a dictionary without triggering a copy.
/// Necessary in Swift 3 but expected to be obsoleted as the way inout works
/// will be changed (eliminating the need for the write-back copy)
func append<K, V>(value: V, toKey key: K, in dict: inout [K : Array<V>]) {
    var a: [V]? = []
    swap(&a, &dict[key])
    a = a ?? []
    a!.append(value)
    swap(&a, &dict[key])
}

/// Removes a value from an array within a dictionary without triggering a copy.
@discardableResult
func remove<K, V>(value: V, fromKey key: K, in dict: inout [K: Array<V>]) -> Bool where V: Equatable {
    var a: [V]? = []
    swap(&a, &dict[key])
    a = a ?? []
    let result = a!.removeElement(value)
    swap(&a, &dict[key])
    return result
}

/// Removes an index from an array within a dictionary without triggering a copy.
func remove<K, V>(index: Int, fromKey key: K, in dict: inout [K: Array<V>]) where V: Equatable {
    var a: [V]? = []
    swap(&a, &dict[key])
    a = a ?? []
    a!.remove(at: index)
    swap(&a, &dict[key])
}