index.js
function addGraph (g1, g2) {
  eachEdge(g2, function (from, to, data) {
    addEdge(g1, from, to, data)
  })
  return g1
}
function addGraph (g1, g2) {
  eachEdge(g2, function (from, to, data) {
    addEdge(g1, from, to, data)
  })
  return g1
}
function addGraph ( g1 , g2 ) { eachEdge ( g2 , function ( from , to , data ) { addEdge ( g1 , from , to , data ) } ) return g1 }
add another subgraph
add another subgraph