Gisi, Mark:

The absence of Public Domain from the license list was not an oversight. A
fair amount of discussion took place to decide how to handle a public domain
designation. The current practice is to create a LicenseRef (a user defined
license reference that is local to an SPDX file).

I think that public domain designations should be handled *exactly* the same
way by SPDX as all other common licenses - just create SPDX license
identifiers for common ones.