pub fn degree(&self, index: usize) -> Option<usize> {
        Some(
            self
                .vertices
                .get(index)?
                .degree()
        )
    }
/// * returns number of vertices adjacent to vertex `index`
/// * `None` if index out of bounds
returns number of vertices adjacent to vertex `index`
`None` if index out of bounds
returns number of vertices adjacent to vertex ` index ` ` None ` if index out of bounds
pub fn degree(&self, index: usize) -> Option<usize> {
        Some(
            self
                .vertices
                .get(index)?
                .degree()
        )
    }
pub fn degree ( & self , index : usize ) -> Option < usize > { Some ( self . vertices . get ( index ) ? . degree ( ) ) }
returns number of vertices adjacent to vertex `index`
 `None` if index out of bounds
returns number of vertices adjacent to vertex ` index ` ` None ` if index out of bounds
&self
index
usize
&self
index
usize