theorem
  H is atomic implies not F is_proper_subformula_of H
proof
  assume H is atomic;
  then H is being_equality or H is being_membership;
  then H = (Var1 H) '=' Var2 H or H = (Var1 H) 'in' Var2 H by Th36,Th37;
  hence thesis by Th67,Th68;
end;
