/// The error type for fallible `MatrixGraph` operations.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum MatrixError {
    /// The `MatrixGraph` is at the maximum number of nodes for its index.
    NodeIxLimit,

    /// The node with the specified index is missing from the graph.
    NodeMissed(usize),
}

pub fn try_add_node(&mut self, weight: N) -> Result<NodeIndex<Ix>, MatrixError>

pub fn try_update_edge(
        &mut self,
        a: NodeIndex<Ix>,
        b: NodeIndex<Ix>,
        weight: E,
    ) -> Result<Option<E>, MatrixError>

pub fn try_remove_edge(&mut self, a: NodeIndex<Ix>, b: NodeIndex<Ix>) -> Option<E>

pub fn add_or_update_edge(
        &mut self,
        a: NodeIndex<Ix>,
        b: NodeIndex<Ix>,
        weight: E,
    ) -> Result<Option<E>, MatrixError>

pub fn get_node_weight(&self, a: NodeIndex<Ix>) -> Option<&N>

pub fn get_node_weight_mut(&mut self, a: NodeIndex<Ix>) -> Option<&mut N>

pub fn get_edge_weight(&self, a: NodeIndex<Ix>, b: NodeIndex<Ix>) -> Option<&E>

pub fn get_edge_weight_mut(&mut self, a: NodeIndex<Ix>, b: NodeIndex<Ix>) -> Option<&mut E>