use "Metis_" prefix rather than "Metis" structure;
the prefix can then be used not only for structures but also signatures and functors. A few other changes were made to the script, to eliminate the need for "metis_env.ML".
(*<*)
theory fakenat imports Main begin;
(*>*)
text{*\noindent
The type \tydx{nat} of natural
numbers is predefined to have the constructors \cdx{0} and~\cdx{Suc}. It behaves as if it were declared like this:
*}
datatype nat = 0 | Suc nat
(*<*)
end
(*>*)