World: r4wp

No.

No

Ok. :-)

It is because the series values actually are a contiguous concept, 
but their respective indices are not a contiguous space of integers.

Which is, frankly, awful when having to compute with indices with 
integers.

... when having to compute indices with integers*

Numbering positions in a series is, in other words, characterized 
as "mathematically modelling 'positions' in a series". Your "inbetween 
positions" are something that does not exist in the series in fact.

Even if you talk about "inbetween", there is _no_ integer in between 
1 and 2, but there is an integer between -1 and 1.

Tail position is inbetween, underlying implementation details cannot 
change that fact.

Tail position is inbetween - actually not. You can write: INSERT 
TAIL SERIES #"a". You do not insert the character "inbetween", in 
fact.

Also, the SKIP function demonstrates the same behaviour quite well 
and consistently.

tail position

 is actually a misnomer, in as far as it corresponds to no proper 
 index of a series. The special behaviour series seen in some functions 
 when operating on series in "tail position" would warrant "tail mode" 
 as a more sensible description of the state the series is in.

as it corresponds to no proper index of a series

 - I reserve the right to disagree. INDEX? TAIL gives some correspondence

Yes, it gives an index that can I'd consider "improper" for the series, 
in as far as the series does not contain a value at that position.

that is arguable, while the INDEX? TAIL is not arguable, giving actual 
correspondence

A rather meaningless correspondence, though.

Subjective interpretations of series tail position are possible, 
as TAIL position exists without pointing to any value.

(And an index that does not correspond to a value in the series, 
at this point in time.)

It is not meaningless as far as I am concerned. Reson: it would be 
meaningless only if you agreed that INDEX? should not yield a value 
for TAIL

Actually, I can demonstrate that even past-tail indices are meaningful 
for blocks, and I did.

Actually, what I was trying to say is that the discussion of "tail 
position" unnecessarily clouds the discussion.

http://www.rebol.com/r3/docs/concepts/series-traversing.html


The first position of the block is called its head. This is the position 
occupied by the word red. The last position of the block is called 
its tail. This is the position immediately after the last word in 
the block. If you were to draw a diagram of the block, it would look 
like this: [...] Notice that the tail is just past the end of the 
block.

Too bad the images are missing...

Well, that is arguable....

(I meant that as a reaction to Andreas' contrib above)

Yes, that's certainly arguable. I say that because I think that "head 
position" and "tail position" sound like dual concepts, whereas they 
are not.

Image is here:
http://www.rebol.com/docs/core23/rebolcore-6.html#section-1.1

The first position of the block is called its head.
 - that is actually false

Hasn't Carl wrote that? :-)

(It would be true only if we defined "the first position" to be compatible 
with the sentence)

Or, in a special case.

(like one example...)

Frankly, I do not care who wrote what.

Hehe. Note that it's "the first position" with plain english first, 
not "the <tt>first</tt> position" with first as reference to the 
FIRST native.

the first position of the block is called its head

 is true if we define "the first position is the position with INDEX? 
 = 1"

Yes.

Which differs from the notion used by FIRST.

which is what I meant by "It would be true if we defined..."

To be clear we should write "the position of the block with INDEX? 
= 1 is called its head"

Or we should come up with better nomenclature :)

Any other consistent nomenclature would do as well.

>> head? []          
== true
>> tail? []
== true
>> first []
** Script Error: Out of range or past end
** Where: halt-view
** Near: first []

Do you consider R2's nomenclature to be particularly consistent and 
simple?

I think you (Ladislav) generally reduce R2's nomenclature to an offset-based 
interpretation?

(i.e. "SKIP-based")

Which differs from the notion used by FIRST.
 You lost me there...how so?

The first position of the block is called its head

 is only true if we define "the first position" as the position with 
 INDEX? = 1.


FIRST, on the other hand, does not follow this "the first position" 
interpretation, but rather gives the value at the current position, 
where current position is the value returned by INDEX?.

I do not, I am just sure that the nomenclature discussed above *is 
inconsistent*

In effect, we have to different notions of "first position": one 
used in HEAD, one used in FIRST.

two* different notions

also, yet another inconsistency (PICK help string):

Returns the value at the specified position in a series.