docs: entailment needs validity

docs/model.tex

@@ -117,6 +117,7 @@ We can now define \emph{semantic} logical entailment.
 \forall \rs \in \monoid.\; 
 \forall \venv \in \Sem{\vctx},\;
 \\&
+n \in \mval(\rs) \land
 n \in \Sem{\vctx \proves \prop : \Prop}_\venv(\rs)
 \Ra n \in \Sem{\vctx \proves \propB : \Prop}_\venv(\rs)
 \end{aligned}