jnd-au

// Given an array collection (may be a Swift Array or an Objective-C NSArray)
// and a function that optionally maps each element to its corresponding group,
// return a Swift Dictionary of the mapped elements in their groups.
// 
// Usage:
// func keyFunc(o: V) -> K {...}
// let grouped = groupBy(keyFunc)([V])
//
// Example:
// func length(s: String) -> Int { return countElements(s) }