how to go from a list of lists to a list of first elements

This is about the question of Doerthe
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Question:

http://ppr.cs.dal.ca:3002/n3/editor/s/Mut5EohI

Would we use built-ins here?
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So how to go from a list of lists to a list of first elements?

We could use list:map like is done in
http://ppr.cs.dal.ca:3002/n3/editor/s/xrtMG2Xc

Jos

-- https://josd.github.io