fn flatten(self) -> ParallelFlatten<'env, Self>
    where
        Self::Item: IntoParallelIterator<'env>,
    {
        let env = self.get_env();
        ParallelFlatten::new(self, env)
    }
// `flatten()` needs to acquire env from outer iterator and thus need to be implemented
// separately.
`flatten()` needs to acquire env from outer iterator and thus need to be implemented
separately.
` flatten () ` needs to acquire env from outer iterator and thus need to be implemented separately .
fn flatten(self) -> ParallelFlatten<'env, Self>
    where
        Self::Item: IntoParallelIterator<'env>,
    {
        let env = self.get_env();
        ParallelFlatten::new(self, env)
    }
fn flatten ( self ) -> ParallelFlatten < ' env , Self > where Self :: Item : IntoParallelIterator < ' env > , { let env = self . get_env ( ) ; ParallelFlatten :: new ( self , env ) }
`flatten()` needs to acquire env from outer iterator and thus need to be implemented
separately.
` flatten () ` needs to acquire env from outer iterator and thus need to be implemented separately .
self
self