How to use the vega-dataflow.derive function in vega-dataflow

pulse.visit(pulse.ADD, function(t) {
      var dt = derive(t);
      lut[tupleid(t)] = dt;
      out.add.push(dt);
    });

pulse.visit(pulse.SOURCE, function(t) {
    var arrays = fields.map(function(f) { return f(t); }),
        maxlen = arrays.reduce(function(l, a) { return Math.max(l, a.length); }, 0),
        i = 0, j, d, v;

    for (; i

pulse.visit(pulse.SOURCE, function(t) {
    for (var i=0, d; i