() : 1
[] : 2

# (map (@ 1 0-0)
#   (.sequence
#     (nothing (@ 1 0-1))
#   )
#   (.result
#     (string (@ 1 5-6) ‘1’)
#   )
# )
# (map (@ 2 0-0)
#   (.sequence
#     (nothing (@ 2 0-1))
#   )
#   (.result
#     (string (@ 2 5-6) ‘2’)
#   )
# )
# :1:0–1: error: empty unordered subsequence
#  () : 1
#  ^
# :2:0–1: error: empty alternation
#  [] : 2
#  ^