theorem
  cdif(f1+r(#)f2,h).(n+1).x = cdif(f1,h).(n+1).x + r* cdif(f2,h).(n+1).x
proof
  set g=r(#)f2;
  cdif(f1+r(#)f2,h).(n+1).x = cdif(f1,h).(n+1).x
       + cdif(g,h).(n+1).x by DIFF_1:22
    .= cdif(f1,h).(n+1).x + r* cdif(f2,h).(n+1).x by DIFF_1:21;
  hence thesis;
end;
