docs: private boolean isPresent(JMFSchema schema) throws JMFSchemaIdException {
      long id = schema.getID();
      JMFSchema reg = (JMFSchema)schemaTable.get(id);
      if (reg != null) {
        // We are assuming that collisions on key don't happen
        if (!schema.equals(reg)) {
          // The schema id is a 64bit SHA-1 derived hashcode, so we really don't expect
          // to get random collisions.
          throw new JMFSchemaIdException("Schema id clash: id=" + id);
        }
        Object newAssoc = schema.getJMFType().getAssociation();
        Object oldAssoc = reg.getJMFType().getAssociation();
        if (newAssoc != oldAssoc && newAssoc != null) {
          if (oldAssoc != null)
            associations.remove(oldAssoc);
          reg.getJMFType().updateAssociations(schema.getJMFType());
          associations.put(newAssoc, reg);
        }
        return true;
      }
      else
        return false;
    }