theorem
  f1 is_right_convergent_in x0 & f2 is_right_convergent_in x0 & (for r
  st x0<r ex g st g<r & x0<g & g in dom(f1-f2)) implies f1-f2
is_right_convergent_in x0 & lim_right(f1-f2,x0)=(lim_right(f1,x0))-(lim_right(
  f2,x0))
proof
  assume that
A1: f1 is_right_convergent_in x0 and
A2: f2 is_right_convergent_in x0 and
A3: for r st x0<r ex g st g<r & x0<g & g in dom(f1-f2);
A4: -f2 is_right_convergent_in x0 by A2,Th53;
  hence f1-f2 is_right_convergent_in x0 by A1,A3,Th54;
  thus lim_right(f1-f2,x0)=lim_right(f1,x0)+lim_right(-f2,x0) by A1,A3,A4,Th54
    .=(lim_right(f1,x0))+-lim_right(f2,x0) by A2,Th53
    .=(lim_right(f1,x0))-lim_right(f2,x0);
end;
