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

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