The contrast between "s" and retroflex "ṣ" in Sanskrit offers a particularly clear case of the domain and duplication problem. The rule that neutralizes it - the so-calles "ruki" rule - shows some complications of exceptional interest [...]. The "ruki" rule retroflexes "s" to "ṣ" after "r", velars, and nonlow vowels.
- Kiparsky (2004), Pag. 168