theorem
  cdif(r1(#)f1-r2(#)f2,h).(n+1).x =
  r1* cdif(f1,h).(n+1).x - r2* cdif(f2,h).(n+1).x
proof
  set g1=r1(#)f1;
  set g2=r2(#)f2;
  cdif(r1(#)f1-r2(#)f2,h).(n+1).x = cdif(g1,h).(n+1).x
       - cdif(g2,h).(n+1).x by DIFF_1:23
    .= r1* cdif(f1,h).(n+1).x - cdif(g2,h).(n+1).x by DIFF_1:21
    .= r1* cdif(f1,h).(n+1).x - r2* cdif(f2,h).(n+1).x by DIFF_1:21;
  hence thesis;
end;
