In extending a previous question, Shaded multicols, how can I put rounded corners on the multicol environment, for the outer 4 corners (ie the corners directly next to the page margins)
This question is only really relevant when multicol shading is used, or multicol borders are used, a solution contained in part I of this question (see above link).
So main thing is that the environment needs to be able to page break mid way through. David Carlisle has suggested in one of the comments to patch the box command, but how can this be done to only round the left 2 corners for a left column, the right 2 corners for a right column and, no corners in a middle column (when there are 3 or more columns).