theorem
  z = <*x0,y0*> & f is_hpartial_differentiable`21_in z implies SVF1(1,
  pdiff1(f,2),z) is_differentiable_in x0
proof
  assume that
A1: z = <*x0,y0*> and
A2: f is_hpartial_differentiable`21_in z;
  consider x1,y1 such that
A3: z = <*x1,y1*> and
A4: ex N being Neighbourhood of x1 st N c= dom SVF1(1,pdiff1(f,2),z) &
ex L,R st for x st x in N holds SVF1(1,pdiff1(f,2),z).x - SVF1(1,pdiff1(f,2),z)
  .x1 = L.(x-x1) + R.(x-x1) by A2;
  x0 = x1 by A1,A3,FINSEQ_1:77;
  hence thesis by A4,FDIFF_1:def 4;
end;
