$$ \begin{array}{l} \lambda f:\,(A \rightarrow A). \lambda x:A. \\ \quad f\ (f\ x) \end{array} \quad \quad \begin{array}{l} \lambda f:\,\downarrow(A^+ \rightarrow\, \uparrow\!A^+). \lambda x:A^+. \\ \quad\mathtt{let\ val}\ y\ =\ (\mathtt{force}\ f)\ x\ \mathtt{in} \\ \quad\mathtt{return}\ (\mathtt{force}\ f)\ y \end{array} $$