theorem Th7:
  z = <*x0,y0*> & f is_hpartial_differentiable`21_in z implies
  hpartdiff21(f,z) = diff(SVF1(1,pdiff1(f,2),z),x0)
proof
  set r = hpartdiff21(f,z);
  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;
  consider N being Neighbourhood of x1 such that
A5: N c= dom SVF1(1,pdiff1(f,2),z) and
A6: 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 A4;
A7: x0 = x1 by A1,A3,FINSEQ_1:77;
  then
A8: SVF1(1,pdiff1(f,2),z) is_differentiable_in x0 by A5,A6,FDIFF_1:def 4;
  consider L,R such that
A9: 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 A6;
  r = L.1 by A2,A3,A5,A9,Def8;
  hence thesis by A5,A9,A7,A8,FDIFF_1:def 5;
end;
