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

pulse.visit(pulse.MOD, function(t) {
      out.mod.push(rederive(t, lut[tupleid(t)]));
    });
  }

paths.forEach(p => {
      values.push(rederive(t, ingest(as != null ? {[as]: p} : p)));
    });
  });

function prepare(source) {
  const item = rederive(source, {});

  if (item.stroke) {
    item.strokeOpacity = 1;
  }

  if (item.fill) {
    item.fillOpacity = INSIDE_OPACITY;
    item.stroke = '#000';
    item.strokeOpacity = 1;
    item.strokeWidth = 2;
  }

  return item;
}