def export! version_number
    version = self.v version_number
    pre_version = self.v version_number - 1
    raise VersionNotExported.new("Previous version not exported.") unless pre_version.exported?
    if f = do_export( pre_version, version)
      version.update_attributes(:exported => true)
    else
      false
    end
  end


## Spec

  it "should change the exported to true" do
    c = comments(:current)
    v1, v2 = versions(:current_v1), versions(:current_v2)
    c.should_receive(:do_export).with(v1, v2).and_return(true)
    c.export!(2)
    v2.should be_exported
  end