gistfile1.txt

PreconditionViolation <- subclass Error {
    constructor { |this.|
        this.name := "PreconditionViolation"
        this.message := "precondition violation"
        stackFrames <- (new Error).stack.split "\n"
        stackFrames.splice 1 3
        this.stack := stackFrames.join "\n"
    }
}

require <- { |condition|
    if (! condition) { throw: new PreconditionViolation }
}

fib <- { |n|
    require: >= n 0
    if (< n 2) {1} { + (fib: - n 1) (fib: - n 2) }
}