Fix use of undefined universe "U"

Fixes #28

docs/model.tex

@@ -152,7 +152,7 @@ The remaining domains are interpreted as follows:
 \Sem{\type \to \type'} &\eqdef& \Sem{\type} \nfn \Sem{\type} \\
 \end{array}
 \]
-For the remaining base types $\type$ defined by the signature $\Sig$, we pick an object $X_\type$ in $\cal U$ and define
+For the remaining base types $\type$ defined by the signature $\Sig$, we pick an object $X_\type$ in $\COFEs$ and define
 \[
 \Sem{\type} \eqdef X_\type
 \]