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 Th146:
  variables_in Ex(x,H) = variables_in H \/ {x}
proof
  thus variables_in Ex(x,H) = variables_in All(x,'not' H) by Th140
    .= variables_in 'not' H \/ {x} by Th142
    .= variables_in H \/ {x} by Th140;
end;
