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Fibonacci numbers:

[

    numeral = @top => [

        zero = @ numeral => [

            case = @ zero => \ z => \ s => z,

            succ = @ zero => (zero.case := \ z => \ s => s zero).val := zero.val + 1,

            val = @ zero => 0,

            pred = @ this => this.case (numeral.zero) (\ x => x),

            add = @ this => \ that => this.case (that) (\x => x.add (that.succ))

        ],

        fib = @ numeral => \ n => n.case (numeral.zero) (\ x => x.case (n) (

            \y => (numeral.fib x).add (numeral.fib y)))

    ],

    main = @ top => ((top.numeral.fib) (top.numeral.zero.succ.succ.succ.succ.succ.succ.succ.succ.succ.succ.succ.succ.succ.succ.succ)).val

].main