I think the answer to Jane’s dilemma is more of an ethical one. Yes, it would be easy to incorporate her melody into a different set of lyrics (who would know?). However (and this only Jane knows), would this melody exist without using her co-writer’s lyrics? Not only for inspiration, but lyrics sets the meter, phrasing, and note duration in a song. I compose from lyrics quite a bit, and they have a significant influence on the music. So, it boils down to what Jane thinks is fair.

John wink