Records an include and does not follow it.
The right resolver when the include graph is somebody else's problem: a prover
handed the file will resolve its own includes, and reading them here would be
work done twice and a chance to disagree about what Axioms/SET007+0.ax means.
Declining is not a failure, so no diagnostic is raised. Tptp.Unit still records
the directive, so Tptp.File.includes/1 reports what was skipped.