function selectionTuples(array, base) {
    return array.map(x => extend(base.fields ? {
      values: base.fields.map(f => getter(f)(x.datum))
    } : {
      [SelectionId]: $selectionId(x.datum)
    }, base));
  }
Maps an array of scene graph items to an array of selection tuples.
@param {string} name  - The name of the dataset representing the selection.
@param {string} base  - The base object that generated tuples extend.

@returns {array} An array of selection entries for the given unit.