# bvar, codition, and domainofapplication

From: Andreas Strotmann <Strotmann@rrz.uni-koeln.de>
Date: Tue, 13 May 2003 19:11:36 +0200
Message-ID: <3EC12748.4010308@rrz.uni-koeln.de>
To: Andreas Strotmann <Strotmann@rrz.uni-koeln.de>, www-math@w3.org

Hello,

even if these comments may be a bit too late, I feel it would be useful
to supplement the recommendations I made during the last few days with
this much more fundamental observation on the use of the main
MathML-Content qualifiers, as an explanation of the rationale behind
many of my recommendations.

I'm sorry that this turned out to be such a long comment, but the matter
feels important to me -- important to get it right in the second
edition, that is.  I hope this helps!

This of course represents my personal analysis, and though I'm pretty
confident, I'm not 100% sure I've got it right.  Comments are therefore
more than welcome.

-- Andreas

There appears to be some confusion still about how to use the new
domainofapplication element.  In some places it is clearly marked as a
qualifier that generalizes the interval qualifier to general sets, but
in some places (such as in this appendix) it appears to be treated as a
regular constructor, thus generalizing the interval *constructor*, which
then brings up what its definition might be.

It is evident to me that in the minds of those who came up with this
important addition to MathML, this element was meant to be used
exclusively as a qualifier (and emphatically not as a constructor) to
generalize the interval *qualifier* (and in my stated opinion in fact to
supercede this dangerous dual use of the interval element), and to
factor out the use of the condition qualifier that allows it to contain
a set, not just a predicate.

In the comments that I have been submitting during the past week or
so, I have therefore assumed that the intention implicit in the choice
of qualifiers available for general use is as follows:

bvar - there may be zero(!) or more of these anywhere; they denote
bound variables in a particular order.

uplimit, lowlimit, interval, and domainofapplication - these all
represent sets that restrict the domain of a variable. Thus,
only the limits (one or both) *or* interval *or*
domainofapplication may appear in a sensible apply.

condition - this one is meant to represent a predicate over the
bound variables of the surrounding element which acts as a
filter defining the set of combinations of bvar values
that the operator ranges over.

Since domainofapplication is new, condition, interval, and the two
limits covered its use in earlier versions, which leads to a need to
deprecate the 'double entendre' usage of these older elements that are
more clearly represented by domainofapplication.  I therefore recommend
deprecating the following in order to clean up the spec:

- the use of a condition qualifier that contains the representation
of a set rather than a predicate -- markup that does this should
simply replace the condition with the domainofapplication
qualifier.

- the interval qualifier (but not the constructor) -- markup that
uses the interval qualifier should replace it with a
domainofapplication qualifier that contains an interval
constructor element with the same content as the original

(Incidentally, this deprecation also makes it easier again to
understand how an interval *constructor* would need to be
allowed to contain a single child element, a condition, because
doing the same thing with an interval *qualifier* seemed rather
spurious because condition should be used instead.)

With these changes in place, we have a clean separation between the uses
of the condition and the domainofapplication qualifiers, as follows:

-  the condition contains a predicate over all the bvars, and may in
principle be used with an empty set of bvars (in which case it
becomes a predicate over the free variables).  The predicate is
in effect the characteristic predicate for a domainofapplication.

-  the domainofapplication qualifier, by contrast, contains a set.
In the current definition, there are two cases that need to be
distinguished:

a) the signature   op: (bvar,dom-of-app,any) -> any
b) the signature   op: (dom-of-app,function) -> any

Of these, a) means the same as

op(bvar, condition(bvar \in dom-of-app), arg)

while b) is equivalent to

op(newvars, dom-of-app, apply(function, newvars))

Note that in the latter case, the domain of application may
actually be a cartesian product in order to assign ranges to
several arguments of the argument function simultaneously.

This is currently not true in a), but it could be specified
in the MathML spec that domainofapplication should contain a
matching-arity cartesian product in the case of multiple
bvars with one domainofapplication (although the fact that it
should contain a cartesian product semantically does not mean
that it would necessarily contain one syntactically -- but if
it does, its meaning is clear).

Note also that a) means the same as

op(dom-of-app,lambda(bvar,arg))

while  a signature of  op(bvars, condition, arg) is equivalent to

op(
dom-of-app(set(bvars,condition,cartesianproduct(bvars)),
lambda(bvars,arg)
)

and a signature like   op(bvars,arg)   means the same as

op(lambda(bvars,arg))

For the signatures in appendix C (and the corresponding text in the main
body), all of this translates to a few general rules:

- lowlimit, uplimit, (deprecated interval), and domainofapplication
belong in one and the same group of qualifiers, the most general
of which is domainofapplication.  They are characterized by not
specifying the names of the variables that they constrain.

- the second group of qualifiers is the condition qualifier in its
reduced definition that requires a predicate argument and disallows
a set argument.  It is characterized by specifically naming the
variables it constrains.

- thus, only one of these groups of qualifiers may be used in an
element, but not both.

- in cases where operators usually take bvar qualifiers, as in
int, diff, product, sum, exists, forall, lack of bvars on first
sight would automatically mean that the single argument must be a
function, as in apply(int,f) and in apply(diff,f).
(Incidentally, this rule does not apply to the lambda constructor
since it does not have an operator.)

But in the case of n-ary constructors, this is problematic (this
includes the set and matrix constructors and the min and max
operators, all of which have versions without any qualifiers
with an n-ary regular function interpretation, and a version with
qualifiers interpreted like int and sum or lambda.)

The problem is that in the current MathML spec, operators like min
and max (and there are plenty of those) are implicitly 'lifted' to
their 'big' version (like \wedge -> \bigwedge , i.e. or -> exists
in a well known notation, or max -> sup):

'small' signature (A*) -> B

'big' signature  (function) -> B
(domainofapplication, function) -> B
(bvar,A) -> B
(bvar,domainofapplication,A) -> B
(bvar,condition,A) -> B

In the future, a general-purpose piece of markup for this lifting
could be useful, so that one could say, for example

apply(big(max),my_real_fn)

apply(max,my_real_fn)

which clearly doesn't do the same thing.

Fortunately, by supplying an empty domainofapplication, this
semantic lifting can be implemented in MathML in a systematic way
without introducing the 'big' operator:

apply(or,domainofapplication(),my_predicate_fn)

can be defined consistently in MathML to have the required meaning.
In other words, we can reconcile the 'big' and 'small' signatures
into a single one by providing the following combined signature:

'combi' signature  (A*) -> B
(domainofapplication,function) -> B
(bvar,A) -> B
(bvar,domainofapplication,A) -> B
(bvar,condition,A) -> B

Replacing the 'function' with its signature, (A -> B), we get

'combi' signature  (A*) -> B
(domainofapplication,(A->B)) -> B
(bvar,A) -> B
(bvar,domainofapplication,A) -> B
(bvar,condition,A) -> B

which is unfortunately a bit different from the

'big' signature ((A->B)) -> B
(domainofapplication,(A->B)) -> B
(bvar,A) -> B
(bvar,domainofapplication,A) -> B
(bvar,condition,A) -> B

From the perspective of automatic, semantically correct processing
of MathML-Content trees, the crucial question is how to reliably
distinguish these two cases when all you have is a user-defined
symbol as an operator (say).  Which of these classes does it
belong to?  How do you interpret  op(a)  compared with
op(bvar(x),b)  safely when you cannot know the signature?

The practical solution is to always assume the 'combi' signature.
This means that apply(product,a) is interpreted the same as
apply(plus,a), and that  apply(plus,bvar(x),f)  is interpreted the
same as apply(product,bvar(x),f), etc.

This means that the indefinite integration and differentiation
operators are the only ones that violate this pattern; user-
defined operators of this sort should always be used with a
possibly empty domainofapplication qualifier when they want to
express application to a function (as in 'prime'):  the completely
qualifier-free use of my_diff_op as in apply(my_diff_op,f) does
*not* encode  f^\myprime.  Instead, I would have to use
apply(my_diff_op,domainofapplication(),f) to get the desired
meaning, unless one is certain that one never would want to
use it with a bvar qualifier.  (Note that it is not necessary
to change the signatures of the existing MathML operators, because
for these, the signatures do exist.)

This also means that the quantifiers, and the product and sum
operators *do* have a reasonable meaning when used without any
qualifiers, and can be assigned the appropriate signature copied
from the appropriate logical connectives or from times and plus,
resp.

Conversely, it means that all n-ary operators have a reasonable
meaning when used with these qualifiers, and their signatures
in appendix C should reflect that fact.

Last not least, it means that apply can be given a general
signature, too:

apply:   ((any*->any), any*)                       -> any
(((any->any)->any),dom-of-app,(any->any)) -> any
(((any->any)->any),bvar,any)              -> any
(((any->any)->any),bvar,dom-of-app,any)   -> any
(((any->any)->any),bvar,condition,any)    -> any

where

dom-of-app:: domainofapplication|uplimit|lowlimit
|lowlimit,uplimit

(leaving out the questionable interval qualifier).

Still, it might be useful to provide a type attribute
in MathML to signal that a csymbol is meant to follow the
pattern of the int and diff operators rather that of the
max and min operators, just to be on the safe side.

Received on Tuesday, 13 May 2003 13:11:40 UTC

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