# `:defl`, `?+`, "wutlus" `{$deft p/wing q/twig r/(list (pair twig twig))}`

Switch with default value.

Identical to `?-` except for that it takes a default case `q` if none of the
cases in `r` match the type of `p`. More specifically, the labels in `q` must
match the span of `p`. As with `?-`, the most common type used to switch on is the cube, which begins with `%` followed by text.

Examples:

    ?-  p
      p.i.r      q.i.r
      p.i.t.r    q.i.t.r
      p.i.t.t.r  q.i.t.t.r
      *          q
    ==
    ~zod/try=> 
      =cor  |=  typ/@ta
            ?+  typ  0
            %a  1
            %b  2
            ==
    new var %cor
    ~zod/try=> 
      (cor 'a')
    1
    ~zod/try=> 
      (cor 'c')
    0

Here is a simple example of `?+` showing that although the cases must
match the span of its input, all possible cases don't need to be covered
since `?+` has a default case. We create a core, `cor` that takes an
input `typ`, a `@ta`. Calling `cor` selects one of our cases when it is
covered, or the default.

    ~zod/try=> 
      ?+  'a'  0
          %a  1
          ==
    1
    ~zod/try=> 
      ?+  [0 'a']  0
          %a  1
          ==
    ! poke-mack-fail
    ! /~zod/try/~2014.11.2..16.45.31..7b2a:<[1 1].[3 7]>
    ! mint-vain
    ! ford: call ~hidper-sommur
    ! poking %poke-txt
    ! error in app %shell on ~zod at instance /shell/terminal
    ~zod/try=> 
      ?+  [0 1]  0
          [0 %a]  1
          ==
    0

This example shows how `?+` can fail. Our input span must match the span
of our cases.