D0 = dictionary:new().
[]
> D1 = dictionary:add(joe, 42, D0).
> D2 = dictionary:add(mike, 41, D1).
[{mike,41},{joe,42}]
> D3 = dictionary:add(robert, 43, D2).
> dictionary:lookup(joe, D3).
{value,42}
> dictionary:lookup(helen, D3).
...