theorem Th11:
  u = <*x0,y0,z0*> & f is_hpartial_differentiable`12_in u implies
  hpartdiff12(f,u) = diff(SVF1(2,pdiff1(f,1),u),y0)
proof
    set r = hpartdiff12(f,u);
    assume that
A1: u = <*x0,y0,z0*> and
A2: f is_hpartial_differentiable`12_in u;
    consider x1,y1,z1 being Real such that
A3: u = <*x1,y1,z1*> & ex N being Neighbourhood of y1 st
    N c= dom SVF1(2,pdiff1(f,1),u) & ex L,R st for y st y in N holds
    SVF1(2,pdiff1(f,1),u).y - SVF1(2,pdiff1(f,1),u).y1 = L.(y-y1) + R.(y-y1)
    by A2;
    consider N being Neighbourhood of y1 such that
A4: N c= dom SVF1(2,pdiff1(f,1),u) & ex L,R st for y st y in N holds
    SVF1(2,pdiff1(f,1),u).y - SVF1(2,pdiff1(f,1),u).y1 = L.(y-y1) + R.(y-y1)
    by A3;
    consider L,R such that
A5: for y st y in N holds
    SVF1(2,pdiff1(f,1),u).y - SVF1(2,pdiff1(f,1),u).y1 = L.(y-y1) + R.(y-y1)
    by A4;
A6: x0 = x1 & y0 = y1 & z0 = z1 by A1,A3,FINSEQ_1:78;
A7: r = L.1 by A2,A3,A4,A5,Def11;
 SVF1(2,pdiff1(f,1),u) is_differentiable_in y0 by A4,A6,FDIFF_1:def 4;
    hence thesis by A4,A5,A6,A7,FDIFF_1:def 5;
end;
