That’s just a temporary category for Under (re-)construction.
I have been thinking about an alternative for use of upvars in callback blocks.
It’s a little less convenient perhaps, but empty slots may be safer in the end:
Yes - end is reported as input by a Boolean (unevaluated) slot. So normally it would be a predicate. But I wanted to enable users to alternatively enter a positive integer as index of the last item to be reported. A number in a Boolean (unevaluated) slot is interpreted as a reporter of that number, so it must be call-ed; there’s no use for other input at calling time, as all of the required information will already be there.
Are you suggesting something like the following?
Thus the internal function becomes more iterative and less recursive, i noticed that internal procedures, assigned to a variable, are often somewhat slow when compared to external helper procedures. Is there a connection?