SRFI 159 defines "written" and "written-simply" that have the
semantics of "write" and "write-simple" of R7RS-small.

What seems missing from the spec is "written-shared(ly)" with the
semantics of "write-shared".

The sample implementation actually defines "written-shared" and exports it.

It seems to be an accidental oversight not to include "written-shared"
in the SRFI document.