Unambiguous syntax definitions
Expression 4*2+1 has two derivation trees
Add syntax definitions: lose the ambiguity
<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.