A Letter Tree

ltree([ [b, [a, [r, [k, bark]]]],
        [c, [a, [r, [r, [y, carry]]],
                [t, cat,
                    [e, [g, [o, [r, [y, category]]]]]]]],
        [d, [e, [l, [a, [y, delay]]]]],
        [h, [e, [l, [p, help]]],
            [o, [p, hop,
                    [e, hope]]]],
        [q, [u, [a, [r, [r, [y, quarry]]]],
                [i, [z, quiz]],
                [o, [t, [e, quote]]]]]
      ]).

• Notice first that the general form is that of a list of lists.
• To move from one letter to the next, we move into the next list item, which should be a list. For example, from b to a in bark.
• To search alternative letters we search along the list - for example from b leading to bark to c