theorem
  for t holds t is negative iff ex u st t = 'not' u
proof
  let t;
  thus t is negative implies ex u st t = 'not' u
    proof
    assume t is negative;
    then consider u such that
        A3: t = Polish-unOp(GRZ-symbols, GRZ-arity, 'not').u
      by Th4, POLNOT_1:80;
    take u;
    thus thesis by A3;
    end;
  thus thesis by Th4, POLNOT_1:81;
end;
