The substitution of anaṅ does not optionally take place when the bahuvrīhi samāsa is a name.,
This declares an option, where the last sutra had made the substitution compulsory. As शतधनुः or शतधन्वा, दृढधनुः or दृढधन्वा ॥,
वा सञ्ज्ञायाम्
Adhyāya 5 · Pāda 4 · Rule 133