Transitional prerequisite checkers

Checkers within prerequisites explains how things will work in version 1.2. What about now?

Your facts will be future proof if you follow this rule:

• If you want a function to be treated as a checker, use a predefined checker, wrap your checking form in as-checker or define it with checker or defchecker.

• If you want your function to be treated the same as a literal value (the same way that 1, for example, would be treated) wrap it in exactly.

Don't use bare functions.

Examples:

     (fact 
       (f 1) => 1
       (provided (g (as-checker even?)) => 33)  ; will always work.
     
    (fact 
      (f 1) => 1
      (provided (filter (exactly even?) [4 5]) => [4])  ; will always work.
     
    (fact 
      (f 1) => 1
      (provided (g even?) => 33)  
      ;; In Midje <1.2, this accepts any even argument to g.
      ;; In Midje >= 1.2, this accepts *only* the even? function.