{ NS_AUTOLOCK(mLock);
InMemoryDataSource::Change(nsIRDFResource* aSource,                           nsIRDFResource* aProperty,                           nsIRDFNode* aOldTarget,                           nsIRDFNode* aNewTarget){    NS_PRECONDITION(aSource != nsnull, "null ptr");    if (! aSource)        return NS_ERROR_NULL_POINTER;    NS_PRECONDITION(aProperty != nsnull, "null ptr");    if (! aProperty)        return NS_ERROR_NULL_POINTER;    NS_PRECONDITION(aOldTarget != nsnull, "null ptr");    if (! aOldTarget)        return NS_ERROR_NULL_POINTER;    NS_PRECONDITION(aNewTarget != nsnull, "null ptr");    if (! aNewTarget)        return NS_ERROR_NULL_POINTER;    nsresult rv;    {        NS_AUTOLOCK(mLock);        // XXX We can implement LockedChange() if we decide that this        // is a performance bottleneck.        rv = LockedUnassert(aSource, aProperty, aOldTarget);        if (NS_FAILED(rv)) return rv;        rv = LockedAssert(aSource, aProperty, aNewTarget, PR_TRUE);        if (NS_FAILED(rv)) return rv;    }    // Notify the world    for (PRInt32 i = PRInt32(mNumObservers) - 1; mPropagateChanges && i >= 0; --i) {        nsIRDFObserver* obs = mObservers[i];        // XXX this should never happen, but it does, and we can't figure out why.        NS_ASSERTION(obs, "observer array corrupted!");        if (! obs)          continue;        obs->OnChange(this, aSource, aProperty, aOldTarget, aNewTarget);        // XXX ignore return value?    }    return NS_RDF_ASSERTION_ACCEPTED;}