[Om3] need list2.ocd for MathML group

[David Carlisle]

[I removed member-math at w3.org> from the CC as it's not an open list]

Michael,

> I have been looking at lists as well, and MathML2 had a <selector>
> element that could be used for lists. Therefore we also need one for the
> OM3/MathML3 CDs.

I think things are better with just the two forms. selector was split
into the matrix and vector symbols on the standard grounds that OM
symbols can not be overloaded by arity, however they can be overloaded
by type (arith1.plus means essentially addition in any construct that has
a plus operation).

If we start to split up symbols based on more precise typing than just
arity, it complicates the mapping between pragmatic mathml and
strict-mathml/OpenMath.

<selector>
  <ci>X</ci>
  <ci>i</ci>
</selector>

means the ith component of the thing X, where X can be indexed by a
single integer, so it's probably a list or vector or something of that
sort. If the mapping to OM relies knowing exactly what sort of thing X
is, then it becomes harder, and in this general case, impossible.

I think at most all we need to do is add some words to the description
of vector_selector to say that it is applicable to any construct indexed
by a single integer.
  
David

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