gistfile1.swift

struct Buffer {
    var count: Int
    
    subscript(at: Int) -> Any {
        // get/set from storage
    }
}

struct Buffer <Element> {
    let count: Int
    
    subscript(at: Int) -> Element
        // fetch from storage
    }
}

extension Buffer where Element: Numeric {
    func sum() -> Element {
        var total: Element = 0
        for i in 0..<self.count {
                total += self[i]
            }
        }
        return total
    }
}
let total = numbers.sum()