struct.c:117
static void
rb_struct_modify(s)
    VALUE s;
{
    if (OBJ_FROZEN(s)) rb_error_frozen("Struct");
    if (!OBJ_TAINTED(s) && rb_safe_level() >= 4)
       rb_raise(rb_eSecurityError, "Insecure: can't modify Struct");
}
