The stack contains states, not symbols. A shifted state encodes the shifted terminal PLUS the information what nonterminals that terminal can be part of (in LR(

The stack contains states, not symbols. A shifted state encodes the shifted terminal PLUS the information what nonterminals that terminal can be part of (in LR(1), each of those nonterminals is augmented with its accepted follow-set). Really the only place where the above breaks down is when in the middle of a rule, after shifting a terminal (or "shifting" a nonterminal after reduction; this is called differently in literature but it really behaves like shifting nonterminals) all that context information is dropped under the assumption that it is not needed for the current rule, but is still available deeper on the stack and will resurface after reducing the current rule. At that point, an error would be missing this information.