     return left
 }
 
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            out += "\(last) \\\\\n"
        }
        return out + "\\end{pmatrix}"
    }
    
    func sort(order: (Element, Element) -> Bool) {
        sort { return order($0, $1) }
    }
    
    func combineAll(_ f: (Element, Element) -> Element) -> Element? {

        }
        return (left == nil ? right : left)
    }

    func findAll(_ f: (Element) -> Bool) -> [Int] {
        if self.count == 0 { return [] }
        if self.count < 5 {
