expr -> term + expr
 expr -> term
 
 term -> term * factor
 term -> factor

 factor -> ( expr )
 factor -> integer

 -----------------------

 BNF

 <expr> ::= <term> "+" <expr> |  <term>

 <term> ::= <factor> "*" <term> |  <factor>

 <factor> ::= "(" <expr> ")" |  <integer>

 <integer> :== <digit> | <nonzero-digit> <digits>

 <digits> := <digit> | <digit> <digits>

 <nonzero-digit> := "1" | "2" | "3" |
                    "4" | "5" | "6" |
                    "7" | "8" | "9"

 -----------------------------------------

 <expr> ::= <expr> "+" <expr> | <expr> "*" <expr> | "(" <expr> ")" | <integer>
 
 <integer> :== <digit> | <nonzero-digit> <digits>

 <digits> := <digit> | <digit> <digits>

 <nonzero-digit> := "1" | "2" | "3" |
                    "4" | "5" | "6" |
                    "7" | "8" | "9"

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