Command ->                                       Command ->                              Command ->
Arg S : String                                   Arg S : String                          Arg S : String

S = '+' >>                                       S = <                                   S = <
C_ADD                                                  '+',                                    1..4
                                                       '-',                                  > >> <
S = '-' >>                                             '*',                                    C_ADD,
C_SUBTRACT                                             '/'                                     C_SUBTRACT,
                                                     > >> <                                    C_MULTIPLY,
S = '*' >>                                             C_ADD,                                  C_DIVIDE
C_MULTIPLY                                             C_SUBTRACT,                           >
                                                       C_MULTIPLY,
S = '/' >>                                             C_DIVIDE                          >>
C_DIVIDE                                             >                                   Exit ERR_COMMAND

>>                                               >>                                      /
Exit ERR_COMMAND                                 Exit ERR_COMMAND

/                                                /
