Comparing Schemas across instances

Does anyone have a prebuild query :) that will compare two schemas on different instances? I have to db's, each containing several custom schema, and I'd like a quick and dirty way to find out what's different. Ie, I given db A, and db B, each with schema one, two, three, and four - and assuming A is the priority db, I need to find out what one_at_b needs to achieve parity with one_at_a, etc etc.

I know how to do this using database links, I just am hoping someone else has written the sql <G> THanks.

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