The samāsānta affix does not optionally come after a tat-puruṣa samāsa of the word pathin with nañ ,
This declares an option, where under the last sutra the omission would have been necessary. Thus अपथम् or अपन्थाः ॥ (See 5.4.74 and 2.4.30).,
पथो विभाषा
Adhyāya 5 · Pāda 4 · Rule 72