procedure main()
    s1 := ["the","that","a"]
    s2 := ["frog","elephant","thing"]
    s3 := ["walked","treaded","grows"]
    s4 := ["slowly","quickly"]

    write(amb(!s1,!s2,!s3,!s4))
end

procedure amb(exprs[])
    s := ""
    every e := !exprs do {
        if \c ~== e[1] then fail
        c := e[-1]
        s ||:= e || " "
        }
    return s
end