Function S

fn S(j: &BitSlice<u8, Lsb0>) -> Point

S(j) := GroupHash^P(︀“z.cash:SinsemillaS”, LEBS2OSP32(I2LEBSP32(j)))

S: {0 .. 2^k - 1} -> P^*, aka 10 bits hashed into the group

https://zips.z.cash/protocol/nu5.pdf#concretesinsemillahash