private void writeIndividus(List<Individual> individus,String father, String son) throws IOException{
        fp.push("\t\t<"+father+">\n");
        for(Individual i:individus){
            try{
                fp.push("\t\t\t<"+son+" ref='"+i.recIdNumber+"'></"+son+">\n");
            }
            catch(NullPointerException e){
                fp.push("\t\t\t<"+son+"></"+son+">\n");
            }
        }
        fp.push("\t\t</"+father+">\n");
    }