prefer direct Assumption.add_assms -- avoid term bindings of Proof_Context.add_assms;
(*<*)
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 approximately as if it were declared like this:
*}
datatype nat = zero ("0") | Suc nat
(*<*)
end
(*>*)