theorem
  for t holds t is conjunctive iff ex u, v st t = u '&' v
proof
  let t;
  thus t is conjunctive implies ex u, v st t = u '&' v
  proof
    assume t is conjunctive;
    then consider u, v such that
        A3: t = Polish-binOp(GRZ-symbols, GRZ-arity, '&').(u, v)
      by Th4, POLNOT_1:82;
    take u, v;
    thus thesis by A3;
  end;
  thus thesis by Th4, POLNOT_1:83;
end;
