C -> A | b c A -> d | b eIf you're parsing non-terminal C and the next token is 'b', you don't know whether it's the first or second alternative of C since they both can start with b. If a grammar can generate the same sentence in multiple different ways (with different parse tress) then it is ambiguous. An ambiguity must start with a context clash (but not all context clashes imply ambiguity). To see if a context clash is also a case of ambiguity you would need to follow the alternatives involved in each context clash to see if they can generate the same complete sequence of tokens.
Last updated: 1995-04-05