code: function(sbm) {
      // create the matrix
      var mat = {}
      mat.x = sbm.x
      mat.y = sbm.y
      mat.data = []
      // populate
      for (var i = 0; i < mat.x; i++) {
        mat.data[i] = _.difference(_.range(mat.y), sbm.data[i])
      }
      // return matrix
      return mat
    }
docs: NOT the matrix