Extending functions on heterogeneous lists

I have implemented my own type HList

using the Apocalisp Level Programming Guide . Now I am trying to implement a function with the following signature:

extend : (A => B) => (A ::: R) => (B ::: R)

where
A, B, R <: HList and ':::' is type-level concatenation.

      

        
        
        
      

    

Unfortunately I don't know how to highlight the components A

and the R

input list A ::: R

. What's the best way to do this?

I cannot rely on external libraries like shapeless and I will not use any of the HList features other than adding and extending.