Last implementation on cons cell
peano number
Making the Nil1 an object render the class List1 covariant
Prepend fails the scala's compiler's variance checking
Revert "Prepend fails the scala's compiler's variance checking"

This reverts commit 282574ba6d6d250c8435b07355bd6f41444e2225.
prepend ok
exercise
basic wiring of expressions
Eval arithmetic expression
OO decomposition
Using pattern matching
As case class defined implicitely companion object, we can simplify the
syntax around instanciation of Expr
Definition of eval inside the main Expr
Show
List manipulation
:: associates to the right, thus we can omit parentheses
insertion sort