length(v)=0,
length(p(v,a))=s(0),
length(p(v,b))=s(0),
length(p(v,x))=s(0),
length(p(p(v,a),b))=s(s(0)),
length(p(p(v,f),g))=s(s(0)),
length(p(p(p(v,c),j),g))=s(s(s(0))),
length(p(p(p(v,w),r),q))=s(s(s(0)))