/// Walk to the twin half-edge.
Walk to the twin half-edge.
Walk to the twin half - edge .
pub fn into_twin(mut self) -> Self
    {
        self.as_twin();
        self
    }
pub fn into_twin ( mut self ) -> Self { self . as_twin ( ) ; self }
Walk to the twin half-edge.
Walk to the twin half - edge .
mut self