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4.5.4 Various Other Existing Extensions
Here is a list of other current extensions in Regina. See chapter
on extensions for more information.

    Regina allows the condition NOVALUE to be trapped by method
  CALL, which is not allowed according to the standard.

    This extension will only be available if the extension
  CALL_ON_NOVALUE has been chosen, and the code was compiled with
  certain preprocessor flags set.

    If NOVALUE is being trapped by method CALL, the current
  clause will be completed as if NOVALUE was not trapped at all,
  returning the default value for an unset symbol as variable value.