docs: function dual(cells, vertex_count) {
    if(!vertex_count) {
      return incidence(unique(skeleton(cells, 0)), cells, 0)
    }
    var res = new Array(vertex_count)
    for(var i=0; i<vertex_count; ++i) {
      res[i] = []
    }
    for(var i=0, len=cells.length; i<len; ++i) {
      var c = cells[i]
      for(var j=0, cl=c.length; j<cl; ++j) {
        res[c[j]].push(i)
      }
    }
    return res
  }