Modify iterator above to define a weak version which verifies only that events of type next occur immediately after events of type hasNext(true) and that the result of ‘hasNext’ can change only after next; multiple consecutive occurrences of events of type hasNext are allowed, as long as they are coherent and the iterator is not required to be fully consumed.

Solution

// iterator2: single iterator, weak version

// verifies only that next occurs immediately after hasNext(true) and that the result of hasNext can change only after next
// multiple consecutive occurrences of hasNext are allowed, as long as they are coherent
// the iterator is not required to be fully consumed

Main = Iterator!;
Iterator = (hasNext(true)+ next)* hasNext(false)+;

The definition exploits the prefix closure operator ! for readability; an equivalent, but more involved, specification can be given with the union operator.

Main = hasNext(true)+ (next Main)? \/ hasNext(false)*;