((a,b) \rho (c,d))i((c,d)\rho (e,f))
\Longrightarrow ((a\leq c)i(b\leq d)i(c\leq e)i(d\leq f))
\Longrightarrow (a\leq e)i(b\leq f)
\Longrightarrow ((a,b)\rho (e,f))