sublist@を使う

```// P26 (**) Generate the combinations of K distinct objects chosen from the N
//          elements of a list.
//     In how many ways can a committee of 3 be chosen from a group of 12
//     people?  We all know that there are C(12,3) = 220 possibilities (C(N,K)
//     denotes the well-known binomial coefficient).  For pure mathematicians,
//     this result may be great.  But we want to really generate all the possibilities.
//
//     Example:
//     scala> combinations(3, List('a, 'b, 'c, 'd, 'e, 'f))
//     res0: List[List[Symbol]] = List(List('a, 'b, 'c), List('a, 'b, 'd), List('a, 'b, 'e), ...```
```object P26 {
def combinations[T](n: Int, l: List[T]): List[List[T]] = {
// flatMapSublists is like list.flatMap, but instead of passing each element
// to the function, it passes successive sublists of L.
def flatMapSublists[U](l: List[T], f: (List[T]) => List[U]): List[U] =
l match {
case Nil => Nil
case sublist@(_ :: tail) => f(sublist) ::: flatMapSublists(tail, f)
}
if (n == 0) List(Nil)
else flatMapSublists(l, (sl) => combinations(n - 1, sl.tail).map((c) => sl.head :: c))
}
}
```

### 難しい記述 †

• def flatMapSublists?[U](l: List[T], f: (List[T]) => List[U]): List[U] の[U]の書き方
• sublist@の書き方
• slの定義について

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