eqBox (Split _ t (x,(y,u)),s) (Split _ t' (x',(y',u')),s') =
    do eqBox (t,s) (t',s')
       eq (x,(y,Boxed (u,s))) (x',(y',Boxed (u',s')))
eqBox (Split _ t (x,(y,u)),s) (Split _ t' (x',(y',u')),s') =
    do eqBox (t,s) (t',s')
       eq (x,(y,Boxed (u,s))) (x',(y',Boxed (u',s')))
eqBox (Split _ t (x,(y,u)),s) (Split _ t' (x',(y',u')),s') =
    do eqBox (t,s) (t',s')
       eq (x,(y,Boxed (u,s))) (x',(y',Boxed (u',s')))