Parsing:
   input = concrete syntax of a program, as a string or file
   output = corresponding abstract syntax

example

input
 { cat = 123; dog=(cat-57); print ((dog * 2)+3)}

output
 (SEQUENCE (ASSIGN CAT 
                   (NUMBER 123))
    (SEQUENCE (ASSIGN DOG 
                      (EXPR - 
                            (VARIABLE CAT) 
                            (NUMBER 57)))
       (SEQUENCE (PRINT (EXPR + 
                              (EXPR * 
                                    (VARIABLE DOG) 
                                    (NUMBER 2)) 
                              (NUMBER 3))) 
          (ENDSEQUENCE))))