dr is a pack of (possibly const) integers
for which sizeof...(dr)== M::extents_type::rank() is true,
the rth element is equal to 1, and
all other elements are equal to 0.
In subclauses [mdspan.layout.reqmts] through [mdspan.layout.stride],
let is-mapping-of be the exposition-only variable template defined as follows:
template<class Layout, class Mapping>constexprboolis-mapping-of=// exposition only
is_same_v<typename Layout::template mapping<typename Mapping::extents_type>, Mapping>;