Constructs a new list by peeking elements in <list> at positions defined by <positions> (a list or tree of positions). <positions> supports the syntax of 'expand-lst'. Ex. (posn-match '(10 20 30 40 50 60 70 80 90) '((0 1) 4 (6))) => ((10 20) 40 (60)) Ex. (posn-match '(10 20 30 40 50 60 70 80 90) '(3*(0) 3_6)) => (10 10 10 40 50 60 70)