World: r3wp

ah, you had trouble, because you were removing an element before 
the INDEX?, not after the INDEX?.

>> b: "abc"
== "abc"
>> b: next next b
== "c"
>> clear back b ; does not modify INDEX?
== ""
>> b
== ** Script Error: Out of range or past end
** Near: mold b
>> index? b
== 2 ; works every time

You are also removing an element before the index, but not from the 
beginning. Apparently clearing from somewhere other then the beginning 
of the series breaks the auto-adjusting (which shouldn't be happening 
anyways). PAST? works the same either way.

It was unfortunate to adjust the result of INDEX? function, since 
we actually obtain a wrong value this way. Auto-adjusting is not 
broken in the case Henrik posted.

>> b: "abc"
== "abc"
>> b: next next b
== "c"
>> index? b
== 3
>> clear back b
== ""
>> index? b
== 2

Nevertheless, as said above, I consider the auto-adjustment idea 
"broken".

past?: func [
	"Returns TRUE if a series index is past its tail."
	series [series! gob! port!]
][
	(index? :series) = (index? back :series)
]

; Note: INDEX? doesn't stay consistent with past-tail references 
in R2.

correction:

past?: func [
	"Returns TRUE if a series index is past its tail."
	series [series! gob! port!]
][
	and~ not same? head? :series :series
		(index? :series) = (index? back :series)
]

; Note: INDEX? doesn't stay consistent with past-tail references 
in R2.

Yet another variant:

past?: func [
	"Returns TRUE if a series index is past its tail."
	series [series! gob! port!]
][
	and~ (index? :series) = (length? head :series)
		not same? tail? :series :series
]

; Note: INDEX? doesn't stay consistent with past-tail references 
in R2.

past?: func [
	"Returns TRUE if a series index is past its tail."
	series [series! gob! port!]
][
	and~ (index? :series) = (length? head :series)
		not same? tail :series :series
]

; Note: INDEX? doesn't stay consistent with past-tail references 
in R2.

(the last is a correction of the one preceding it)

Simplification:

past?: func [
	"Returns TRUE if a series index is past its tail."
	series [series! gob! port!]
][
	not same? :series skip :series 0
]

; Note: INDEX? doesn't stay consistent with past-tail references 
in R2.

Note: I am not sure, whether this would work with ports and gobs, 
though, since I do not know, whether SAME? behaves similarly for 
them

Ladislav, it needs to work in the case of auto-adjusting indexes 
too.

Auto-adjusting is broken in the case that Henrik mentioned. As a 
separate issue, the case Henrik mentioned has *better* behavior, 
because the whole idea of auto-adjusting is a bad one, so it not 
working is an improvement.

But because of that, the PAST? function for R2 needs to work whether 
the index auto-adjusts or not. So before you submit another version, 
test it against past-tail references with auto-adjusting indexes. 
If it works then, it works.

To get a past-tail reference that auto-adjusts, clear from the head 
of the series rather than from further along.

why didn't you try? it works

every past-tail series auto-adjusts in ceratain interpreter versions 
(in other no past-tail series auto-adjusts), no exceptions

Henrik does not know any series that does not auto-adjust, it is 
just your error

(you just need to read above)

Note: all versions work just in (auto-adjusting) R2

I am mentioning that, since there were some versions of R2 that did 
not auto-adjust

(long time ago)

But, it is possible to write a modified version, which would work 
regardless of auto-adjustment

In 2.7.7, indexes auto-adjust if you clear the series from the beginning, 
but not if you clear from later on. In the same interpreter.

Wrong, see above

You did not check Henrik's example as I did

I did, but got different results than I am getting now.

Actually, you did not, it was just an error

With the same code, no less, I am getting different resulta now.

I didn't post the the old results.

You are getting the same results you got before, it was just your 
error, resulting from the fact, that auto-adjustment adjusts past-tail 
series to tail indices, i.e. to 2 in case Henrik posted

Nevertheless, it is easy to write code that would work regardless 
of auto-adjustment

(which means, it would work in R2, old R2, and R3)

Well, I don't know why the results are different from the same code, 
but your code works now, so if it's alright with you I'll update 
R2/Forward with it.

I'll just chock up the old results to a momentary glitch until they 
recur.

E.g. this modification works regardless of auto-adjustment: (i.e. 
even in R3)

past?: func [
	"Returns TRUE if a series is past its tail."
	series [series! gob! port!]
] [
	and~ greater-or-equal? index? :series index? tail :series
	    not same? :series tail :series
]

but, pick whichever version you like

BTW, "returns true if a series index is past its tail" mixes different 
datatypes - series index is integer, while tail is a series, so, 
my opinion is, that the shorter wording is less misleading

The not same? :series skip :series 0 line works well enough, afaict. 
Do you have a case for which it doesn't work and the larger test 
is required?

aha, it looks, that it works even in R3...

Case for which it does not work... - only in ancient versions of 
R2

nevertheless, I see it as a bug, that it works even in R3

That is actually an auto-adjustment test, and I thought, that no 
auto-adjustment was taking place in R3

Nevermind, I will declare it "a feature"

Well, in R3 PAST? is built-in. And auto-adjustment was taken away 
in R3 on purpose, at the same time that PAST? was added.

Yes, but skip :series 0 auto-adjusts still, but, as I said, I do 
not mind

How about the help text? The longer one (currently used in R3) looks 
misleading.

(comparing series index - an integer, to series tail - a series)

It's not misleading without the auto--adjustment. With the auto-adjustment 
a past-tail index will auto-adjust, which should be mentioned somehow.