Share 'How to extract an edge curve of surfaces'

hello all, i'm quite a new both for this forum and grasshopper, even with some experiences with rhinoscript. i'm now testing the grasshopper's ability compared with scripting. So, i have a basic question. How can we extract an edge curve from a surface ? it's sometimes necessary to make shells from complex surfaces to compensate offset commands. i know this could be solved with making such curv…