As a result of the wonders of technology and social media I can "talk" to the nice woman (don't know her name) who asked the intelligent question about painting leaves that seem to overlap one another using negative painting. Hello there, hope you enjoyed the demo!... I directed her to this blog and the demo last January about painting clematis leaves. She needs to scroll down past the demo of daffodil leaves to the first demo on the blog. I hope she has a look and finds it helpful.
Above is a painting I did years ago showing leaves bottom left painted this way.