data Element (a : Set) : Set where
  cons : a @$\rightarrow$@ Maybe (Element a) @$\rightarrow$@ Element a

datum : {a : Set} @$\rightarrow$@ Element a @$\rightarrow$@ a
datum (cons a _) = a

next : {a : Set} @$\rightarrow$@ Element a @$\rightarrow$@ Maybe (Element a)
next (cons _ n) = n

record SingleLinkedStack (a : Set) : Set where
  field
    top : Maybe (Element a)