code: public Extension getExtension(ObjectIdentifier oid) {
          if (extensions == null)
              return null;
  
          // following returns null if no such OID in map
          //XXX consider cloning this
          return extensions.get(OIDMap.getName(oid));
      }
docs: get an extension
  
  @param oid ObjectIdentifier of extension desired
  @returns Extension of type <extension> or null, if not found