$\forall$$A_{1}$,$A_{2}$:Type, $B_{1}$:($A_{1}$$\rightarrow$Type), $B_{2}$:($A_{2}$$\rightarrow$Type).
\\[0ex]strong{-}subtype($A_{1}$; $A_{2}$)
\\[0ex]$\Rightarrow$ ($\forall$$a$:$A_{1}$. subtype\_rel($B_{1}$($a$); $B_{2}$($a$)))
\\[0ex]$\Rightarrow$ subtype\_rel(fpf($A_{1}$; $a$.$B_{1}$($a$)); fpf($A_{2}$; $a$.$B_{2}$($a$)))