if (nsnull == mStackArena) mStackArena = new StackArena();
if (!mStackArena) { mStackArena = new StackArena(); if (!mStackArena) return NS_ERROR_OUT_OF_MEMORY; }
PresShell::PushStackMemory(){  if (nsnull == mStackArena)     mStackArena = new StackArena();     return mStackArena->Push();}