private void modify()
{
// if readonly then throw readonly exception
if (m_isReadonly)
throw ReadonlyErr.make("Map is readonly").val;
// if we have a cached m_readonlyMap, then detach
// it so it remains m_immutable
if (m_readonlyMap != null)
{
m_readonlyMap.m_map = cloneMap(m_map);
m_readonlyMap = null;
}
}