Limit values for the instances

[damianc]

For example, one instance can get at most 1 value in a row, then another must do it. Until it is done, the instance cannot get any value.

genA.next();
// 1

genB.next();
// 2

genB.next();
// 2 or Error

genA.next();
// 3

genA.next();
// 3 or Error

genB.next();
// 4

Of curse, it must not be the default behavior. Rather than that, it should be achieved by something like an operator:

genA.group(genB, 'switch') ;
// group, cluster, merge, join or like that