# Functions
NewSimpleGraph creates and returns a SimpleGraph.
# Variables
ErrParallelEdge is returned when an operation tries to create a disallowed parallel edge.
ErrSelfLoop is returned when an operation tries to create a disallowed self loop.
ErrVertexNotFound is returned when an operation is requested on a non-existent vertex.
# Structs
SimpleGraph is a mutable, non-persistent undirected graph.