reserve k,m,n for Element of NAT,
  i, j for Nat,
  a, b, c for object,
  X, Y, Z for set,
  D, D1, D2 for non empty set;
reserve p, q, r, s for FinSequence;
reserve t, u, v, w for GRZ-formula;

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;
