app(v,v)=v, app(v,p(v,a))=p(v,a), app(p(v,a),p(v,b))=p(p(v,b),a), app(p(p(v,a),f),p(v,j))=p(p(p(v,j),a),f), app(p(v,j),p(p(v,f),k))=p(p(p(v,f),k),j)