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;
