$\forall$$T$:Type. 
\\[0ex]subtype\_rel($T$; $\mathbb{Z}$)
\\[0ex]$\Rightarrow$ ($\forall$${\it bs}$,${\it as}$:($T$ List). no\_repeats($T$; ${\it as}$) $\Rightarrow$ sorted(${\it as}$) $\Rightarrow$ no\_repeats($T$; merge(${\it as}$; ${\it bs}$)))