code: function getExpression(_, ctx, params) {
    if (_.$params) { // parse expression parameters
      parseParameters(_.$params, ctx, params);
    }
    var k = 'e:' + _.$expr + '_' + _.$name;
    return ctx.fn[k]
      || (ctx.fn[k] = accessor(parameterExpression(_.$expr, ctx), _.$fields, _.$name));
  }
docs: Resolve an expression reference.