This restriction does not apply to suffixes denoting unsaturation, such as "-ene" and "-yne" (see R-3.1), which are not regarded as principal characteristic groups; nor to the ionic and radical suffixes, such as "-ium", "-ide", and "-yl" (see R-, which may be combined and may also be added to certain principal characteristic group suffixes, for example, ethanaminium and pyridin-1-ium-3-yl.

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