Roget's Thesaurus for the iPhone

I can't find it, and it's driving me crazy.

I don't want a thesaurus in dictionary form. If you've never experienced the joy of browsing a schema-based thesaurus, you really haven't lived.

You don't have to understand the Dewey Decimal system to understand that once you've found the library section on cooking, if you wander in one direction you find your way to hunting books, and past that you'll discover books on horticulture and gardening. The other direction? Kitchen remodeling, home improvement, housekeeping.

See how easy that was? You started looking for one concept and found closely related ones nearby. That's what a real Roget's Thesaurus is like.

For example, pulling from the classic 1911 edition:

Entry #444 contains synonyms for spectator
#445: Optical instruments
#446: Visibility
#447: Invisibility
#448: Appearance (including a cross-reference back to #443, imperfect vision)

And so forth. A dictionary-style thesaurus can never capture the raw power intrinsic to this structure.

Please, someone, anyone? I could write one myself, theoretically, but it's the licensed content that would really make it tick. Content from 1911, while free of copyright, wouldn't be nearly as useful as a modern data set.