code: def move_edge_source(self, edge_id, node_a, node_b):
          
          # Grab the edge
          edge = self.get_edge(edge_id)
  
          # Alter the vertices
          edge['vertices'] = (node_b, edge['vertices'][1])
  
          # Remove the edge from node_a
          node = self.get_node(node_a)
          node['edges'].remove(edge_id)
  
          # Add the edge to node_b
          node = self.get_node(node_b)
          node['edges'].append(edge_id)
docs: Moves an edge originating from node_a so that it originates from node_b.