subtree = g_ptr_array_index(tree_stack,tree_stack->len-1);
g_assert(subtree != NULL);
g_ptr_array_remove_index(tree_stack,tree_stack->len-1);
subtree = g_ptr_array_index(tree_stack,tree_stack->len-1);
g_assert(subtree != NULL);
g_ptr_array_remove_index(tree_stack,tree_stack->len-1);