We will encode a mini-AST in Prolog using complex data structures. A "node" will consist of either a nb(Functor,LeftExpr,RightExpr), nu(Functor,Expr) or nn(Number).

nb(Functor,LeftExpr,RightExpr) -- "node binary", Functor is guaranteed to be a binary arithmetic predicate that can be evaluated with `is`. LeftExpr and RightExpr are recursively defined "nodes".

nu(Functor,Expr) -- "node unary", Functor is guaranteed to be a unary arithmetic predicate that can be evaluated with `is`. Expr is a recursively defined "node".

nn(Number) -- "node number", Number is guaranteed to be just that.

Hence, the following AST
      +
     / \
    *   random
   / \       \
  2  3        5
would be encoded as nb(+,nb(*,nn(2),nn(3)),nu(random,nn(5))).