theorem
  f1 is_hpartial_differentiable`11_in z0 & f2
  is_hpartial_differentiable`11_in z0 implies pdiff1(f1,1)-pdiff1(f2,1)
is_partial_differentiable_in z0,1 & partdiff(pdiff1(f1,1)-pdiff1(f2,1),z0,1) =
  hpartdiff11(f1,z0) - hpartdiff11(f2,z0)
proof
  assume that
A1: f1 is_hpartial_differentiable`11_in z0 and
A2: f2 is_hpartial_differentiable`11_in z0;
A3: pdiff1(f1,1) is_partial_differentiable_in z0,1 & pdiff1(f2,1)
  is_partial_differentiable_in z0,1 by A1,A2,Th9;
  then
  partdiff(pdiff1(f1,1)-pdiff1(f2,1),z0,1) = partdiff(pdiff1(f1,1),z0,1) -
  partdiff(pdiff1(f2,1),z0,1) by PDIFF_1:31;
  then
  partdiff(pdiff1(f1,1)-pdiff1(f2,1),z0,1) = hpartdiff11(f1,z0) - partdiff
  (pdiff1(f2,1),z0,1) by A1,Th13
    .= hpartdiff11(f1,z0) - hpartdiff11(f2,z0) by A2,Th13;
  hence thesis by A3,PDIFF_1:31;
end;
