if \1
  <letter \1> : "\1"
else if \2
  <letter \2> : "\2"
end if
<letter a> : "a"

# (if (@ 1 0-2) ‘\1’
#   (.inner
#     (map (@ 2 2-2)
#       (.sequence
#         (keys (@ 2 2-13) ‘<letter \1>’)
#       )
#       (.result
#         (string (@ 2 16-20) ‘"\1"’)
#       )
#     )
#   )
#   (.otherwise
#     (if (@ 3 3-7) ‘\2’
#       (.inner
#         (map (@ 4 4-4)
#           (.sequence
#             (keys (@ 4 4-15) ‘<letter \2>’)
#           )
#           (.result
#             (string (@ 4 18-22) ‘"\2"’)
#           )
#         )
#       )
#       (.otherwise nil)
#     )
#   )
# )
# (map (@ 6 0-0)
#   (.sequence
#     (keys (@ 6 0-10) ‘<letter a>’)
#   )
#   (.result
#     (string (@ 6 13-16) ‘"a"’)
#   )
# )