def <<(patch)
      known_patch = @patches[patch.name]
      if known_patch.nil?
        @patches[patch.name] = patch
      elsif patch.content != known_patch.content
        raise ContradictionError, [known_patch, patch]
      end
    end
def <<(patch)
      known_patch = @patches[patch.name]
      if known_patch.nil?
        @patches[patch.name] = patch
      elsif patch.content != known_patch.content
        raise ContradictionError, [known_patch, patch]
      end
    end
Adds a patch to the set. Raises ContradictionError in case
 if patch set already contains a patch with the same name and
 different content.

 patch :: Patch object to add