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 atomic or t is negative or t is conjunctive or t is being_equality
proof
  let t;
  set s = Polish-WFF-head t;
  assume A1: not thesis;
  then not s in GRZ-ops by ENUMSET1:def 1;
  then A2: s in VAR by XBOOLE_0:def 3;
  s = GRZ-symbols-head t by POLNOT_1:def 33;
  hence contradiction by A1, A2, Th5, POLNOT_1:69;
end;
