$\forall$$T$:Type, $a$,${\it a'}$:$T$, $b$,${\it b'}$:($T$ List).
\\[0ex](cons($a$; $b$) = cons(${\it a'}$; ${\it b'}$)) $\Leftarrow\!\Rightarrow$ guard((($a$ = ${\it a'}$) $\wedge$ ($b$ = ${\it b'}$)))