List can't be Tuple

We can't afford to have WHICH based on all the elements of List, for two
reasons: it's slow, and its .elems is forcing eagerness that causes the
self-reference check to bring on heat-death of the universe.

src/core/List.pm

@@ -615,12 +615,6 @@ my class List does Iterable does Positional { # declared in BOOTSTRAP
         self.new(|c);
     }
 
-    multi method WHICH(List:D:) {
-        $!WHICH //= self.^name
-          ~ '|'
-          ~ (^self.elems).map( {'(' ~ self[$_].VAR.WHICH ~ ')'} ).join;
-    }
-
     method is-lazy() {
         if $!todo.DEFINITE {
             $!todo.reify-until-lazy();