a. The coronis is not written when the rough breathing stands on the first word: ὁ ἄνθρωπος ἅ_νθρωπος.
b. Crasis does not occur when the first vowel may be elided. (Some editors write τἄλλα, etc.)
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].