Explain the reason for calling do_with_enum without a block [ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52592 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/set.rb

@@ -151,7 +151,7 @@ class Set
       @hash.replace(enum.instance_variable_get(:@hash))
       self
     else
-      do_with_enum(enum)
+      do_with_enum(enum)  # make sure enum is enumerable before calling clear
       clear
       merge(enum)
     end