/// <summary>helper routine to clone a new production part adding a given label
/// </summary>
protected virtual production_part add_lab(production_part part, string lab)
{
/* if there is no label, or this is an action, just return the original */
if (lab == null || part.is_action())
return part;
/* otherwise build a new one with the given label attached */
return new symbol_part(((symbol_part) part).the_symbol(), lab);
}
