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 Th22
    .= r1* cdif(f1,h).(n+1).x + cdif(g2,h).(n+1).x by Th21
    .= r1* cdif(f1,h).(n+1).x + r2* cdif(f2,h).(n+1).x by Th21;
  hence thesis;
end;
