external_prover: "exec" the command line, in order to preserve the exact process context of the "system" invocation (this recovers interruptibility of E-1.0, which assumes to be the process group leader);
(*<*)
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
(*>*)