public ISemantReturn<ImmutableList<Tuple<Env, ExternDecln>>> GetExternDecln(Env env) {
var declns = Semant(GetDeclns, ref env);
var externDeclns = declns.ConvertAll(_ => Tuple.Create(_.Item1, _.Item2 as ExternDecln));
return SemantReturn.Create(env, externDeclns);
}
}