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Grammar For Expression:
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prog:	(expr NEWLINE)* ;
expr:	expr ('*'|'/') expr
    |	expr ('+'|'-') expr
    |	INTEGER
    |	'(' expr ')'
    ;
INTEGER : [0-9]+  ;
NEWLINE : [\r\n]+ ;
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SAMPLE INPUT = 48 + 2 * 3
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AST OUTPUT:
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prog
      -> \n
      -> expr
              -> expr
                      -> expr
                              -> 3
                      -> *
                      -> expr
                              -> 2
              -> +
              -> expr
                      -> 48