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
  H is existential implies H = Ex(bound_in H,the_scope_of H)
proof
  assume
A1: H is existential;
  then ex x st H = Ex(x,the_scope_of H) by Th43;
  hence thesis by A1,Th43;
end;
