mean.swift

    static func mean<S: Sequence, T>(
        of sequence: S,
        oneScalar: T,
        scale: (T, S.Element) -> S.Element
    ) -> S.Element? where S.Element: AdditiveArithmetic, T: FloatingPoint {
                
        var it = sequence.makeIterator()
        
        guard let initial = it.next() else {
            return nil
        }
        
        var count = oneScalar
        var mean = initial
        
        while let x = it.next() {
            mean = mean + scale(T(1) / count, x - mean)
            count += oneScalar
        }
  
        return mean
    }