public extension Collection {
  func inits<T>(_ transform: (SubSequence) throws -> T) rethrows -> [T] {
    try (0...count).map { try transform(prefix($0)) }
  }

  func inits() -> [SubSequence] {
    inits { $0 }
  }
}

print([1, 7, 14, 9, 11].inits { $0.reduce(0, +) })