code: static boolean isBlankNode(Object v) {
          // Note: A value is a blank node if all of these hold true:
          // 1. It is an Object.
          // 2. If it has an @id key its value begins with '_:'.
          // 3. It has no keys OR is not a @value, @set, or @list.
          if (v instanceof Map) {
              final Map<String, Object> map = (Map<String, Object>) v;
              if (map.containsKey("@id")) {
                  return ((String) map.get("@id")).startsWith("_:");
              } else {
                  return map.isEmpty() || !map.containsKey("@value") || map.containsKey("@set")
                          || map.containsKey("@list");
              }
          }
          return false;
      }
docs: Returns true if the given value is a blank node.
  
  @param v
  the value to check.
  
  @return true if the value is a blank node, false if not.