$ java GathererBasics      (JDK 25)
windowFixed(3) of 1..7 = [[1, 2, 3], [4, 5, 6], [7]]
CHECK ok   : windowFixed(3) groups into [1,2,3][4,5,6][7], last window is the short remainder
CHECK ok   : windowFixed(0) throws IllegalArgumentException: 'windowSize' must be greater than zero
windowSliding(3) of 1..5 = [[1, 2, 3], [2, 3, 4], [3, 4, 5]]
CHECK ok   : windowSliding(3) of 5 elements produces exactly 3 overlapping windows, each length 3
CHECK ok   : windowSliding(3) on a 2-element stream still emits one short window: [[1, 2]]
CHECK ok   : windowSliding(3) on an empty stream emits nothing
fold building a String: [1][2][3][4]
CHECK ok   : fold("", acc+"[n]") over 1..4 builds "[1][2][3][4]" left to right
CHECK ok   : fold's output stream has exactly one element, the final accumulation
scan running total of 1..5 = [1, 3, 6, 10, 15]
CHECK ok   : scan(0, +) over 1..5 emits every prefix sum: [1,3,6,10,15]
fold(0,+) over 1..3   = [6]
scan(0,+) over 1..3   = [1, 3, 6]
CHECK ok   : fold(0,+) over 1..3 emits just the final sum [6]
CHECK ok   : scan(0,+) over 1..3 emits every partial sum [1,3,6]
exit=0