theorem
for i be set st i in dom G
  & f1 is_partial_differentiable_in x,i
  & f2 is_partial_differentiable_in x,i
 holds f1-f2 is_partial_differentiable_in x,i
     & partdiff(f1-f2,x,i)=partdiff(f1,x,i)-partdiff(f2,x,i)
proof
   let i0 be set;
   assume A1: i0 in dom G;
   set i=In(i0,dom G);
   assume A2: f1 is_partial_differentiable_in x,i0
        & f2 is_partial_differentiable_in x,i0;
   (f1-f2)*reproj(i,x) = f1*reproj(i,x)-f2*reproj(i,x) by A1,Th26;
   hence f1-f2 is_partial_differentiable_in x,i0 by A2,NDIFF_1:36;
   thus partdiff(f1,x,i0)-partdiff(f2,x,i0)
     = diff(f1*reproj(i,x)-f2*reproj(i,x),proj(i).x) by A2,NDIFF_1:36
     .= partdiff((f1-f2),x,i0) by A1,Th26;
end;
