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Ambiguous Syntax Definitions

Expression 4*2+1 has two derivation trees!

Unambiguous definition

<expression > ::=

<expression ><lowop > <term > |

<term >

<term > ::= <term > <highop > <factor > | <factor >

<factor > ::= <numeral > | ( <expression > )

<lowop > ::= + | -

<highop > ::= * | /

Extra level of structure makes derivation unique but syntax complicated.