Raw input, a given:

   { fact = 1;
     i = 1;
     while (i < 7) do
     { fact = (fact * i);
       i = (i + 1) }
     print f }




First stage:  seems trivial but it's important

   '{', ' ', 'f', 'a', 'c', 't', ' ', '=', '1', ';', ' ', 'i', ' ', '=', ..., '}', EOF




Second stage:

   "{", punctuation, 0
   "fact", identifier, 0
   "=", operator, 0
   "1", number, 1
   ...
   "while", reserved_word, 0
   ...
   "^D", end_of_file, 0              Lexical analysis




Third stage:   Parsing

   -------------------------
   | block | | | | | | | | |
   ----------|---|---|---|--
             |   |   |   |
             |   |   |   v
             |   |   |  -------------
             v   v   |  | print | | |
                     |  ----------|--
             .   .   |            |
             .   .   |            v
             .   .   |           ---------------------
                     |           | variable | "fact" |
                     |           ---------------------
                     v
                    -----------------
                    | while | | | | |
                    ----------|---|--
                                  |
                                  v
                                 -----------------
                                 | block | | | | |
                                 ----------|---|--
                                           |   |
                                           |   |
                                           |   v
                                           |   
                                           |   .
                                           |   .
                                           |   .
                                           |   
                                           v    
                                          ---------------------------
                                          | assignment | "fact" | | |
                                          ------------------------|--
                                                                  v
                                                                 ----------------------------
                                                                 | expression | "*" | | | | |
                                                                 ---------------------|---|--
                                                                                      |   |
                                                                                      |   |
                                                                                      |   |
                                                                                      v   v




Fourth stage, could be compilation: turn that into assembly language,
           or could be interpretation: a big recursive function that does what the tree says.