docs: public Label getLabel(String expr) {
          if(expr==null)  return null;
          expr = hudson.util.QuotedStringTokenizer.unquote(expr);
          while(true) {
              Label l = labels.get(expr);
              if(l!=null)
                  return l;
  
              // non-existent
              try {
                  labels.putIfAbsent(expr,Label.parseExpression(expr));
              } catch (ANTLRException e) {
                  // laxly accept it as a single label atom for backward compatibility
                  return getLabelAtom(expr);
              }
          }
      }