Recursive use of elements <option> and <optional>

In docbook 4.x it was possible to use <optional> in a recursive way, eg: 
https://www.postgresql.org/docs/current/static/sql-createtable.html or 
https://www.postgresql.org/docs/current/static/sql-select.html. Optional 
elements often are only valid under the additional demand, that they are 
part of another optional element. The same holds true for arguments of 
commands or procedures.

We try to upgrade to docbook 5.x, but the schema doesn't allow the 
recursive use of <option> and <optional>. Do you have an advice how to 
express such situations? Do you have plans to modify the docbook 5 
schema to allow recursive use of the two elements?

Kind Regards
Jürgen Purtz
[email protected]