docs: function boundary(cells) {
    var res = []
    for(var i=0,il=cells.length; i<il; ++i) {
      var c = cells[i]
      for(var j=0,cl=c.length; j<cl; ++j) {
        var b = new Array(c.length-1)
        for(var k=0, l=0; k<cl; ++k) {
          if(k !== j) {
            b[l++] = c[k]
          }
        }
        res.push(b)
      }
    }
    return normalize(res)
  }