Grouping items in Sequence

public extension Sequence {
    func group<T>(by key: (Element) -> T) -> [T: [Element]] {
        Dictionary(grouping: self, by: key)
    }
    
    func groupCustom<T>(by modifier: (Element) -> T) -> [T: [Element]] {
        var result: [T: [Element]] = [:]
        for element in self {
            let key = modifier(element)
            result[key, default: []] += [element]
        }
        return result
    }
}