The a of ac (añc ) is elided at the end of a bha aṅga (stem).,
Thus दधीचः पश्य, दधीचा, दधीचे, मधूचः पश्य, मधूचा, मधूचे ॥ The lengthening of the first member takes place by 6.3.138: and न् of अञ्च is elided by 6.4.24.,
अचः
Adhyāya 6 · Pāda 4 · Rule 138