Upcasting of ADT instance is required before calling a method

[Timon555]

In the book I am translating, I encountered the following problem.

We need to upcast "Suc{x}" to "Unary(Suc{x})" in order to call the Add function: "Unary(Suc{x}).Add(y)".
Here is the declaration of the Add function: "pure func (x Unary) Add(y Unary) Unary".

This is also the case with other functions and ADTs.