An ordered list of state operations emitted by one node task.
Operations apply in emission order. combine is sequential composition: a.combine(b) applies a's operations and then b's, so for a StateKey.replace key the later value wins and for an operation-valued key both operations apply. Updates from different tasks in one superstep are never combined; the scheduler applies them task by task in frontier order.
Appends a removal of key's stored entry; later reads observe the key's initial value. It does not remove one item from a collection - that is an ordinary typed update.
Appends a removal of key's stored entry; later reads observe the key's initial value. It does not remove one item from a collection - that is an ordinary typed update.