[mdspan.submdspan.submdspan] p4.3 and p5.1 could be removed

The last item of \mandates and
the first item of \expects
are subsumed by the "Effects: Equivalent to:" semantics.

However, the \expects ends with

\begin{note}
These conditions ensure that the mapping returned by \tcode{submdspan_mapping}
matches the algorithmically expected index-mapping given the slice specifiers.
\end{note}

So perhaps the redundancy for \expects is intended.
Although it can be rephrased to allude to subsumed preconditions.

I think the last item of \mandates can still be removed without problems.

Originally https://github.com/cplusplus/draft/pull/6338#discussion_r1240039539.

Search the TeX sources for the [mdspan.submdspan.submdspan] subclause and inspect p4.3 and p5.1 alongside the mandates, expects, and Effects wording. Resolve whether the mandates item is redundant and whether expects needs rephrasing; done when the wording and source text consistently reflect that decision.