Usage
Returns "/sandbox" if the template detects that the page where it is transcluded is a subpage named "sandbox" (i.e. a sandbox page); otherwise it returns nothing.
This can be useful to e.g. switch the version of a template used in its documentation to the version in its sandbox (by replacing {{template name}} on the /doc page with {{template name{{\sandbox}}}}) when that documentation is transcluded on the template's sandbox page.