I think a GPL-like extension of a pencil sharpener would be the following. You develop a new pencil sharpener that copies 90% of the original design but includes a sharper blade that makes nicer pencil tips; when you release that product, you must show everyone how your pencil sharpener is designed and definitely must not prevent anyone from seeing how the original (that you started with) was designed.
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The GPL doesn't restrict the products of tools.
I think a GPL-like extension of a pencil sharpener would be the following. You develop a new pencil sharpener that copies 90% of the original design but includes a sharper blade that makes nicer pencil tips; when you release that product, you must show everyone how your pencil sharpener is designed and definitely must not prevent anyone from seeing how the original (that you started with) was designed.