code: public StringClauses append(String key, StringClauses subclauses) {
          Validate.notNull(StringUtil.trimToNull(key), "key must be a non-null, non-empty value");
  
          key = StringUtil.trimToEmpty(key).toLowerCase();
  
          if (clauses.containsKey(key)) {
              throw new IllegalArgumentException("Cannot add clause with key '" + key + "' because it is already defined");
          }
  
          clauses.put(key, subclauses);
          return this;
      }
docs: Adds a new sub-clause at the end of the list with the given key.