Presumably that means "this is not true for any more powerful class of grammars".
Any particular pair grammars from that class. I mean if you showed me a yacc grammar for Pascal and another one for Java, I am sure I could prove they were ineqivelent.
More tricky, if you gave someone clever a hand written Pascal parser and that same yacc grammar she could probably prove equivilency with some effort. But there is no guarantee that this is always possible.
> I mean if you showed me a yacc grammar for Pascal and another one for Java,
This is why I said "any", and not "all". In most practical cases, you can tell if two grammars are different, but it's easy to construct cases where you can't.
Any particular pair grammars from that class. I mean if you showed me a yacc grammar for Pascal and another one for Java, I am sure I could prove they were ineqivelent.
More tricky, if you gave someone clever a hand written Pascal parser and that same yacc grammar she could probably prove equivilency with some effort. But there is no guarantee that this is always possible.