How to use the vega-dataflow.Tuple.set function in vega-dataflow

var o = trans ? {} : item,
      offset = item.mark.def.offset,
      orient = item.mark.def.orient,
      width  = group.width,
      height = group.height; // TODO fallback to global w,h?

  if (dl.isArray(offset)) {
    var ofx = offset[0],
        ofy = offset[1];

    switch (orient) {
      case 'left':   { Tuple.set(o, 'x', -ofx); Tuple.set(o, 'y', ofy); break; }
      case 'right':  { Tuple.set(o, 'x', width + ofx); Tuple.set(o, 'y', ofy); break; }
      case 'bottom': { Tuple.set(o, 'x', ofx); Tuple.set(o, 'y', height + ofy); break; }
      case 'top':    { Tuple.set(o, 'x', ofx); Tuple.set(o, 'y', -ofy); break; }
      default:       { Tuple.set(o, 'x', ofx); Tuple.set(o, 'y', ofy); }
    }
  } else {
    if (dl.isObject(offset)) {
      offset = -group.scale(offset.scale)(offset.value);
    }

    switch (orient) {
      case 'left':   { Tuple.set(o, 'x', -offset); Tuple.set(o, 'y', 0); break; }
      case 'right':  { Tuple.set(o, 'x', width + offset); Tuple.set(o, 'y', 0); break; }
      case 'bottom': { Tuple.set(o, 'x', 0); Tuple.set(o, 'y', height + offset); break; }
      case 'top':    { Tuple.set(o, 'x', 0); Tuple.set(o, 'y', -offset); break; }
      default:       { Tuple.set(o, 'x', 0); Tuple.set(o, 'y', 0); }
    }
  }

  if (trans) trans.interpolate(item, o);

prototype.interpolate = function(item, values) {
  var key, curr, next, interp, list = null;

  for (key in values) {
    curr = item[key];
    next = values[key];
    if (curr !== next) {
      if (skip[key] || curr === undefined) {
        // skip interpolation for specific keys or undefined start values
        Tuple.set(item, key, next);
      } else if (typeof curr === 'number' && !isFinite(curr)) {
        // for NaN or infinite numeric values, skip to final value
        Tuple.set(item, key, next);
      } else {
        // otherwise lookup interpolator
        interp = d3.interpolate(curr, next);
        interp.property = key;
        (list || (list=[])).push(interp);
      }
    }
  }

  if (list === null && item.status === Status.EXIT) {
    list = []; // ensure exiting items are included
  }

data.forEach(function(n) {
    Tuple.set(n, output.x, n.x);
    Tuple.set(n, output.y, n.y);
    Tuple.set(n, output.width, n.dx);
    Tuple.set(n, output.height, n.dy);
    Tuple.set(n, output.depth, n.depth);
  });

data.forEach(function(n) {
    Tuple.set(n, output.x, n.x);
    Tuple.set(n, output.y, n.y);
    Tuple.set(n, output.depth, n.depth);
    if (mode === PARTITION) {
      Tuple.set(n, output.width, n.dx);
      Tuple.set(n, output.height, n.dy);
    }
  });

function set(t) {
    var path = shape(sourceX(t), sourceY(t), targetX(t), targetY(t), tension);
    Tuple.set(t, output.path, path);
  }

function update(d) {
    var v = get(d),
    v = v == null ? null
      : b.start + s * ~~((v - b.start) / s);
    Tuple.set(d, output.start, v);
    Tuple.set(d, output.end, v + s);
    Tuple.set(d, output.mid, v + s/2);
  }
  input.add.forEach(update);

function set(t) {
    var ll = [lon(t), lat(t)];
    var xy = proj(ll) || [null, null];
    Tuple.set(t, output.x, xy[0]);
    Tuple.set(t, output.y, xy[1]);
  }

data.forEach(function(n) {
    Tuple.set(n, output.x, n.x);
    Tuple.set(n, output.y, n.y);
    Tuple.set(n, output.depth, n.depth);
    if (mode === PARTITION) {
      Tuple.set(n, output.width, n.dx);
      Tuple.set(n, output.height, n.dy);
    }
  });

data.forEach(function(n) {
    Tuple.set(n, output.x, n.x);
    Tuple.set(n, output.y, n.y);
    Tuple.set(n, output.depth, n.depth);
    if (mode === PARTITION) {
      Tuple.set(n, output.width, n.dx);
      Tuple.set(n, output.height, n.dy);
    }
  });

ofy = offset[1];

    switch (orient) {
      case 'left':   { Tuple.set(o, 'x', -ofx); Tuple.set(o, 'y', ofy); break; }
      case 'right':  { Tuple.set(o, 'x', width + ofx); Tuple.set(o, 'y', ofy); break; }
      case 'bottom': { Tuple.set(o, 'x', ofx); Tuple.set(o, 'y', height + ofy); break; }
      case 'top':    { Tuple.set(o, 'x', ofx); Tuple.set(o, 'y', -ofy); break; }
      default:       { Tuple.set(o, 'x', ofx); Tuple.set(o, 'y', ofy); }
    }
  } else {
    if (dl.isObject(offset)) {
      offset = -group.scale(offset.scale)(offset.value);
    }

    switch (orient) {
      case 'left':   { Tuple.set(o, 'x', -offset); Tuple.set(o, 'y', 0); break; }
      case 'right':  { Tuple.set(o, 'x', width + offset); Tuple.set(o, 'y', 0); break; }
      case 'bottom': { Tuple.set(o, 'x', 0); Tuple.set(o, 'y', height + offset); break; }
      case 'top':    { Tuple.set(o, 'x', 0); Tuple.set(o, 'y', -offset); break; }
      default:       { Tuple.set(o, 'x', 0); Tuple.set(o, 'y', 0); }
    }
  }

  if (trans) trans.interpolate(item, o);
  return true;
}