Here is a version of basket add-to with a recursive local function for doing the insert, avoiding the clumsy mapcar and "did we insert or not" check copied from the F# code.

  (defmeth basket add-to (me product quantity)
    (labels ((insert (lines)
               (tree-case lines
                 ((line . rest) (if (equal line.product-sku product.sku)
                                  (cons (build-line product
                                                    (+ line.quantity quantity))
                                        rest)
                                  (cons line (insert rest))))
                 (() (list (build-line product quantity))))))
      (new basket lines (insert me.lines))))