let println expr : string =
        let rec print expr =
            match expr with
            | Application(expr1, expr2) ->
                // Function application.
                print expr1 + " " + print expr2

            | SpecificCall <@@ (+) @@> (_, _, exprList) ->
                // Matches a call to (+). Must appear before Call pattern.
                print exprList.Head + " + " + print exprList.Tail.Head

            | Int32(n) -> n.ToString()
            | Lambda(param, body) ->
                // Lambda expression.
                "fun (%s:%s) -> " + param.Name + (param.Type.ToString()) + print body
            | Let(var, expr1, expr2) ->
                let letPart = var.IsMutable |> function
                // Let binding.
                    | true -> "let mutable %s = " + var.Name
                    | _ ->    "let %s = " + var.Name
                letPart + print expr1 + " in " + print expr2
            | PropertyGet(_, propOrValInfo, _) ->
                propOrValInfo.Name
            | String(str) ->
                str
            | Value(value, typ) ->(value.ToString())
            | Var(var) ->
                var.Name
            | _ -> (expr.ToString())
        print expr