if \1
  <letter \1> : "\1"
else
  if \2
    <letter \2> : "\2"
  end if
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
#     (if (@ 4 2-4) ‘\2’
#       (.inner
#         (map (@ 5 4-4)
#           (.sequence
#             (keys (@ 5 4-15) ‘<letter \2>’)
#           )
#           (.result
#             (string (@ 5 18-22) ‘"\2"’)
#           )
#         )
#       )
#       (.otherwise nil)
#     )
#   )
# )