reserve k,m,n for Element of NAT,
  a,X,Y for set,
  D,D1,D2 for non empty set;
reserve p,q for FinSequence of NAT;
reserve x,y,z,t for Variable;
reserve F,F1,G,G1,H,H1 for ZF-formula;
reserve sq,sq9 for FinSequence;

theorem Th56:
  H is negative implies
  (F is_immediate_constituent_of H iff F = the_argument_of H)
proof
  assume H is negative;
  then H = 'not' the_argument_of H by Def30;
  hence thesis by Th52;
end;
