docs: function shift_cell_xls(cell, tgt, opts) {
  	var out = dup(cell);
  	if(tgt.s) {
  		if(out.cRel) out.c += tgt.s.c;
  		if(out.rRel) out.r += tgt.s.r;
  	} else {
  		out.c += tgt.c;
  		out.r += tgt.r;
  	}
  	if(!opts || opts.biff < 12) {
  		while(out.c >= 0x100) out.c -= 0x100;
  		while(out.r >= 0x10000) out.r -= 0x10000;
  	}
  	return out;
  }