support structural composition (THEN_ALL_NEW) for proof methods;
clarified preparation for goal restriction: Goal.conjunction_tac only once;
export Method.parse0, notably for Eisbach;
more explicit type cases_state;
(*<*)
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
(*>*)