If you really really miss the `/` from `Numeric`...

dividable.swift

public protocol Dividable {
    static func / (lhs: Self, rhs: Self) -> Self
}

extension Int : Dividable {}
extension Double : Dividable {}

extension Sequence where Element : Numeric & Dividable {
    func average() -> Element {
        var i: Element = 0
        var total: Element = 0
        
        for value in self {
            total = total + value
            i += 1
        }
        
        return total / i
    }
}