A Real Fix For the "Backslashes in Included Equations" Bug

Ruby's String.sub!(pattern, replacement) routine is fundamentally
broken. But the block version works fine.

Using the broken routine in the Chunk handler was a subtle mistake.

lib/chunks/chunk.rb

@@ -61,7 +61,7 @@ module Chunk
     end
 
     def unmask
-      @content.sub!(mask, @unmask_text)
+      @content.sub!(/#{mask}/){|s| s.replace @unmask_text}
     end
 
     def rendered?
@@ -73,7 +73,7 @@ module Chunk
     end
 
     def revert
-      @content.sub!(mask, @text)
+      @content.sub!(/#{mask}/){|s| s.replace @text}
       # unregister
       @content.delete_chunk(self)
     end