[adrianratnapala]

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.

[wyager]

Yes, that is what I meant.

> 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.