variable.c:437
static int
mark_global_entry(key, entry)
    ID key;
    struct global_entry *entry;
{
    struct trace_var *trace;
    struct global_variable *var = entry->var;

    (*var->marker)(var->data);
    trace = var->trace;
    while (trace) {
	if (trace->data) rb_gc_mark_maybe(trace->data);
	trace = trace->next;
    }
    return ST_CONTINUE;
}
