The definition-list (OR condition) is unclear as to what action should be taken in cases where multiple conditions are matched.
In definition-list, it possibly have to buffer the whole stream to make a decision if it uses a schema that leaves very long conditition in the OR branch. For instance,
A = {'a'};
FINALLY NOT A = {'a'}, 'b';
S = A | FINALLY NOT A;where it need to read and buffer the input aaaaaa...b to the end of the stream to determine whether it matches A or FINALLY NOT A.