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Find keys mapped to a value in Swift dictionary
extension Dictionary where Value: Equatable {
/// Returns all keys mapped to the specified value.
/// ```
/// let dict = ["A": 1, "B": 2, "C": 3]
/// let keys = dict.keysForValue(2)
/// assert(keys == ["B"])
/// assert(dict["B"] == 2)
/// ```
func keysForValue(value: Value) -> [Key] {
return flatMap { (key: Key, val: Value) -> Key? in
value == val ? key : nil
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This is similar to the allKeysForObject(_:) method of NSDictionary, but it is generic so it can return an array of properly types keys, instead of an array of AnyObject.

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