function amb1(sequence sets, object res=0, integer idx=1)
integer ch = 0
integer pass = 0
    if idx>length(sets) then
        pass = 1
    else
        if res=0 then
            res = repeat(0,length(sets))
        else
            ch = sets[idx-1][res[idx-1]][$]
        end if
        for k=1 to length(sets[idx]) do
            if ch=0 or sets[idx][k][1]=ch then
                res[idx] = k
                {pass,res} = amb1(sets,res,idx+1)
                if pass then exit end if
            end if
        end for
    end if
    return {pass,res}
end function

sequence sets = {{"the","that","a"},
                 {"frog","elephant","thing"},
                 {"walked","treaded","grows"},
                 {"slowly","quickly"}}
integer pass
sequence res
    {pass,res} = amb1(sets)
    if pass then
        puts(1,"success: ")
        for i=1 to length(sets) do
            res[i] = sets[i][res[i]]
        end for
        ?res
    else
        puts(1,"failure\n")
    end if