response to Mark Shoulson on optional "cu"

[lojbab (Bob LeChevalier)]

Mark's confusion here may explain why we have to work hard on explaining
how elision works.  His questions are NOT stupid, since many people ask
them.

The change from 'elidable' to 'optional' for "cu" actually has no real
effect on the grammar of valid Lojban sentences.  It does not produce a
shift-reduce error.  We do not accept shift/reduce errors in our
unambiguity proof based on YACC.  But the change IS really tied into the
elidable system, and that is where Mark gets confused.  All of his
problem examples do not parse as he expects, because he omitted critical
elidables.  I am responding on-line rather than with off-line
preparation, so I cannot fiddle with his actual examples.  So pardon if
I use my own.

<le mlatu citka> <le ratcu>

is two sumti with no selbri.  Optional 'cu' has no direct relevance
here.  It will not be inserted by the parser, which accepts a collection
of sumti with no selbri as grammatical (a list of answers to ma
questions, for example)

What the parser WILL do is add in elidable 'ku' at the end of each "le"
description:

(le mlatu citka KU> <le ratcu KU> VAU

Because 'cu' is an optional instead of an elidable, it will never be
inserted using the YACC error processing.  It does not need to be,
because every sentence that uses "cu" has an alternative form using some
other elidable. Modifying the above example to one that makes the
probably intended sense:

le mlatu cu citka le ratcu
The cat eats the rat.

parses correctly, with the "cu" identifying that there was an elidable
'ku' after 'mlatu':

<le mlatu KU> cu <citka {le ratcu KU} VAU>

So the 'real sentence' is
le mlatu ku cu citka le ratcu ku vau

IN THIS SENTENCE, THE "cu" IS 'OPTIONAL'.  It is NOT possible to talk
about optionality in a sentence where you have already elided, because
taking the option changes the conditions for elision.

Thus

le mlatu ku citka le ratcu ku vau

is equally grammatical to the version with "cu" in it, and indeed the
final 'ku' and 'vau' are elidable regardless of the "cu", so

le mlatu ku citka le ratcu
<le mlatu ku> <citka {le ratcu KU} VAU>

works as well as

le mlatu ku cu citka le ratcu
<le mlatu ku> cu <citka {le ratcu KU} VAU>

and in this pair of examples it can be seen that the "cu" is 'optional',
as opposed to 'elidable', since it parses the same with or without the
"cu".

Without the "cu" however, the "ku" after mlatu is NOT elidable, since
the x1 sumti then combines with the selbri to form the 2-sumti phrase
that I started with.

The reason for making the grammar change is simply that the parser never
really needs to insert an elided "cu".  If it gets to the selbri, it
must already have terminated any preceding sumti with their proper
terminators in order to realize that there is a selbri to recognize.  In
such a case it does not need the "cu".

On the other hand, HUMAN BEINGS prefer the "cu" to the rats nest of
elidables.  They know that if they insert the optional "cu", that they
don;t need to worry about terminating a variety of constructs that might
be seen before the selbri.  Many examples of this are shown in draft
textbook lesson 4/4A, though probably not enough or well-explained,
since Mark got confused.

HUMAN listeners also prefer the "cu" becausae it helps them, who parse
more than one token at a time in their minds, to divide the sentence
they hear in two - the pre-selbri part and the selbri+post-selbri part.
In moderately complex sentence, for example with an abstraction clause
having sub arguments in front of the selbri, this makes Lojban
understandable:

le nu le prenu ku klama le zarci kei ku se pluka mi
The event the person goes to the store  pleases  me

Uses 3 elidables and you may have trouble finding the two selbri (main
and abstract) in the text.  But it parses grammatically (unless I made a
screw-up) and effectively identically to the easier to see version using
"cu":

le nu le prenu cu klama le zarci cu se pluka mi
<le {nu (le prenu KU) cu (klama <le zarci KU> VAU) KEI} KU> cu <se pluka {mi} VAU>

with the "cu", two elidables before 'se pluka' became really elidable.
Without the "cu" you need at least 2 of the 4 that were elided, and if
they are the wrong two, you still get in trouble (you must have the
second "ku" as one of the at-least-two , or without the "cu", the "se
pluka" becomes part of a tanru of x1 of the form abbreviated as:

le <nu klama kei> <se pluka>

The key thing to remember is that elidables are NOT always elidable,
they are OPTIONALLY elidable under specific if not well-stated
conditions.  This makes OPTIONAL 'cu' seem not quite so different from
OPTIONALLY elidable 'cu'.  The peril is that a description such as the
grammar change report presumes that the reader knows how each of the
constructs being changed works before and after the change.  In this
case, what really happened was that a path that the parser never needs
to take - inserting an 'elided' "cu" - has been eliminated from the
parser tables.  In some earlier versions of the grammar, we had not been
able to do this.  Presumably the grammar is now stable and well-formed
enough to allow the optional "cu".  The disadvantage of the change,of
course, is that with the "cu" not listed in the elidables, when the
parser processes text, it inserts all the other elidables and not the
"cu", and reading the parser output for a large block of text is more
difficult without that large-print CU to tell you where the selbri is.
This however is a small tradeoff for a cleaner and more honest grammar,
and I still hope to eventually have a post processor that can
automatically process parser outputs into some kind of structured
readble form.  But not this month.

lojbab