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
  x <> y implies (H/(x,y))/(x,z) = H/(x,y)
proof
  assume x <> y;
  then not x in variables_in (H/(x,y)) by Th184;
  hence thesis by Th182;
end;
