code: function getEncode(_, ctx) {
    var spec = _.$encode,
        encode = {}, name, enc;
  
    for (name in spec) {
      enc = spec[name];
      encode[name] = accessor(encodeExpression(enc.$expr, ctx), enc.$fields);
      encode[name].output = enc.$output;
    }
    return encode;
  }
docs: Resolve an encode operator reference.