RE: breadcrumbs for Info . . . . . .

[Drew Adams]

>    Even if the "cache" consisted only of a set of node+parent pairs
>    (regardless of the order among pairs), that would be sufficient.
> 
> A slight twist on that data structure will give us a "reverse trie",
> which is a set of elements (NODE PARENT PARENT^2...), with elements'
> tails sharing storage (i.e, being other elements of the set).

Cf. `Info-build-toc'.