theorem Th67:
  Free All(x,y,p) = Free p \ {x,y}
proof
  thus Free All(x,y,p) = Free All(y,p) \ {x} by Th62
    .= (Free p \ {y}) \ {x} by Th62
    .= Free p \ ({x} \/ {y}) by XBOOLE_1:41
    .= Free p \ {x,y} by ENUMSET1:1;
end;
