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

/                                                /
