variable.c:1358
static int
sv_i(key, value, tbl)
    ID key;
    VALUE value;
    st_table *tbl;
{
    if (rb_is_const_id(key)) {
	if (!st_lookup(tbl, key, 0)) {
	    st_insert(tbl, key, key);
	}
    }
    return ST_CONTINUE;
}
