reserve p,p1,p2,q,r,F,G,G1,G2,H,H1,H2 for ZF-formula,
  x,x1,x2,y,y1,y2,z,z1,z2,s,t for Variable,
  a,X for set;
reserve M for non empty set,
  m,m9 for Element of M,
  v,v9 for Function of VAR,M;
reserve i,j for Element of NAT;

theorem Th176:
  H is biconditional implies H/(x,y) is biconditional
proof
  given H1,H2 such that
A1: H = H1 <=> H2;
  H/(x,y) = (H1/(x,y)) <=> (H2/(x,y)) by A1,Th163;
  hence thesis;
end;
