Calling an operator serially

[Philip Doctor]

I’ve got a KeyedStream, I only want max parallelism (1) per key in the keyed 
stream (i.e. if key is OrganizationFoo then only 1 input at a time from 
OrganizationFoo is processed by this operator).  I feel like this is obvious 
somehow, but I’m struggling to find the docs for this.  Can anyone point me the 
right way here?