assumption
  function digit/1
    \add(\1 "0")
  end function
end assumption

<letter \digit(1)> : "\digit(0)"
<letter \digit(2)> : "\digit(1)"
<letter \digit(3)> : "\digit(2)"
<letter \digit(4)> : "\digit(3)"
<letter \digit(5)> : "\digit(4)"
<letter \digit(6)> : "\digit(5)"
<letter \digit(7)> : "\digit(6)"
<letter \digit(8)> : "\digit(7)"
<letter \digit(9)> : "\digit(8)"
<letter \digit(0)> : "\digit(9)"