if (PR_TOP_IO_LAYER == id) return fd;
if (PR_TOP_IO_LAYER == id) { if (PR_IO_LAYER_HEAD == fd->identity) return fd->lower; else return fd; }
PR_IMPLEMENT(PRFileDesc*) PR_GetIdentitiesLayer(PRFileDesc* fd, PRDescIdentity id){    PRFileDesc *layer = fd;    if (PR_TOP_IO_LAYER == id) return fd;    for (layer = fd; layer != NULL; layer = layer->lower)    {        if (id == layer->identity) return layer;    }    for (layer = fd; layer != NULL; layer = layer->higher)    {        if (id == layer->identity) return layer;    }    return NULL;}  /* PR_GetIdentitiesLayer */