Represents a single license requirement, which must include a valid
LicenseItem, and may allow current and future versions of the license,
and may also allow for a specific exception
A convenience wrapper for a license and optional exception that can be
checked against a license requirement to see if it satisfies the requirement
placed by a license holder
A single license term in a license expression, according to the SPDX spec.
This can be either an SPDX license, which is mapped to a LicenseId from
a valid SPDX short identifier, or else a document AND/OR license ref