def unshift_edge(edge)
	    @vertices.unshift(edge.last) unless edge.last == @edges.first.first
	    @vertices.unshift(edge.first) # unless edge.first == @edges.last.last
	    @vertices.pop if @vertices.last == @vertices.first
	    @edges.unshift(edge)
	end
def unshift_edge(edge)
	    @vertices.unshift(edge.last) unless edge.last == @edges.first.first
	    @vertices.unshift(edge.first) # unless edge.first == @edges.last.last
	    @vertices.pop if @vertices.last == @vertices.first
	    @edges.unshift(edge)
	end
Prepend an {Edge} and its vertices
 @param edge [Edge]	the {Edge} to unshift