The ABNF in Appendix IV of the 2.0 version of the specification allows for short form identifiers, LicenseRef values or combinations to form a license-expression. However the values "NONE" and "NOASSERTION" are not valid in a license expression, despite their useful and defined meaning in the specification.
There are tools that validate their license fields using a license-expression (two such tools being the package managers npm and composer, in JavaScript and PHP, respectfully), making the values "NONE" and "NOASSERTION" invalid.
Are these two values excluded from the ABNF on purpose?
