code: public static String getResponsiveLabelClass(IResponsiveLabel r) {
              if(!shouldRenderResponsiveClasses(r)) {
                  return "";
              }
              
              int  colxs = sizeToInt(getSize(r, Sizes.xs));
  	    int colsm = sizeToInt(getSize(r, Sizes.sm));
              int colmd = sizeToInt(getSize(r, Sizes.md));
              int collg = sizeToInt(getSize(r, Sizes.lg));
  
              StringBuilder sb = new StringBuilder();
  
              if (colmd > 0) {
                  sb.append("col-md-");
                  sb.append(colmd);
                  sb.append(' ');
              }
  
              if (colxs > 0) {
                  sb.append("col-xs-");
                  sb.append(colxs);
                  sb.append(' ');
              }
              if (colsm > 0) {
                  sb.append("col-sm-");
                  sb.append(colsm);
                  sb.append(' ');
              }
              if (collg > 0) {
                  sb.append("col-lg-");
                  sb.append(collg);
                  sb.append(' ');
              }
              if (sb.length() > 0) {
                  return sb.substring(0, sb.length() - 1);
              }
  
              return null;
  	}
docs: Create the responsive class combination
  @param r the component bearing the responsiveness attributes
  @return null, if there's no label-col-xx attribute