assumption
  macro letter/1
    <letter \1> : \1
  end macro
end assumption

# (assumption (@ 1 0-10)
#   (.inner
#     (macro (@ 2 2-7) ‘letter/1’
#       (.inner
#         (map (@ 3 4-20)
#           (.sequence
#             (keys (@ 3 4-15) ‘<letter \1>’)
#           )
#           (.result
#             (string (@ 3 18-20) ‘\1’)
#           )
#         )
#       )
#     )
#   )
# )