A Way of Remembering
lThe combination
 | [ É ]
inside a list Òmelts awayÓ into
  , É
unless É is empty, then it just melts away
lExamples:
l[1 | [2, 3, 4] ] == [1, 2, 3, 4]
l[1, 2 | [ 3 , 4] ] == [1, 2, 3, 4]
l[1, 2, 3 | [4] ] == [1, 2, 3, 4]
l[1, 2, 3, 4 | [ ] ] == [1, 2, 3, 4]
l