fcn comb(k,seq){	// no repeats, seq is finite
   seq=seq.makeReadOnly();	// because I append to parts of seq
   fcn(k,seq){
      if(k<=0)    return(T(T));
      if(not seq) return(T);
      self.fcn(k-1,seq[1,*]).pump(List,seq[0,1].extend)
	   .extend(self.fcn(k,seq[1,*]));
   }(k,seq);
}