if \1
  <letter \1> : "\1"
end if

# (if (@ 1 0-2) ‘\1’
#   (.inner
#     (map (@ 2 2-2)
#       (.sequence
#         (keys (@ 2 2-13) ‘<letter \1>’)
#       )
#       (.result
#         (string (@ 2 16-20) ‘"\1"’)
#       )
#     )
#   )
#   (.otherwise nil)
# )