code: function Edge(properties) {
  
      // Possible Properties
      var edge = {
        id: properties.id,
        type: properties.type || 'undirected',
        label: properties.label || '',
        source: properties.source,
        target: properties.target,
        weight: +properties.weight || 1.0
      };
  
      if (properties.viz)
        edge.viz = properties.viz;
  
      if (properties.attributes)
        edge.attributes = properties.attributes;
  
      return edge;
    }
docs: Edge structure.
  A function returning an object guarded with default value.
  
  @param  {object} properties The edge properties.
  @return {object}            The guarded edge object.