message: Extract derive_esk() into separate function
diff: @@ -346,7 +346,17 @@ impl<E: JubjubEngine> Note<E> {
  // reduce to uniform value
  E::Fs::to_uniform(&buffer[..])
  }
  - Rseed::AfterZip212(rseed) => E::Fs::to_uniform(prf_expand(&rseed, &[0x05]).as_bytes()),
  + Rseed::AfterZip212(_) => self.derive_esk().unwrap(),
  + }
  + }
  +
  + /// Returns the derived `esk` if this note was created after ZIP 212 activated.
  + pub fn derive_esk(&self) -> Option<E::Fs> {
  + match self.rseed {
  + Rseed::BeforeZip212(_) => None,
  + Rseed::AfterZip212(rseed) => {
  + Some(E::Fs::to_uniform(prf_expand(&rseed, &[0x05]).as_bytes()))
  + }
  }
  }
  }
  