CDT: Literals can have the same lexical form but different values.

This means that two RDF terms (`Node`) can be same-term by RDF Term rules but have different values.

Thy can't be safely used as keys into a map or cache.

They may be created in different parser runs even into the same graph.

Encountered in #3570 in the CDT test suite.

Can there be a way to enforce the restrictions of use?

@hartig

Start with the CDT test suite referenced in #3570 and trace how the two RDF terms are created across parser runs. Determine the intended restriction for using Node instances as map or cache keys, then define tests that distinguish same lexical forms from different values and confirm the agreed behavior.

Written by the indexing model from the issue text.