for 0 to 9 as \1
  <letter \add(\1, "0")> : \add(\1, "0")
end for

# (for (@ 1 0-3) ‘0’ ‘9’ (.variable ‘\1’)
#   (.inner
#     (map (@ 2 2-2)
#       (.sequence
#         (keys (@ 2 2-24) ‘<letter \add(\1, "0")>’)
#       )
#       (.result
#         (string (@ 2 27-40) ‘\add(\1, "0")’)
#       )
#     )
#   )
# )