QueryOperatorDef.scala

 lazy val queryOperationDef: PackratParser[Seq[Expression]] =
    "?" ~> repsep(filter | select | top | skip | orderBy, "&")

  lazy val filter: PackratParser[Expression] =
    "$filter" ~> "=" ~> predicate ^^ Filter

  lazy val top: PackratParser[Expression] =
    "$top" ~> "=" ~> number ^^ Top

  lazy val skip: PackratParser[Expression] =
    "$skip" ~> "=" ~> number ^^ Skip

  lazy val orderBy: PackratParser[Expression] = (
      "$orderby" ~> "="~> propertyList <~ "asc" ^^ OrderByAsc
    | "$orderby" ~> "="~> propertyList <~ "desc" ^^ OrderByDesc
    | "$orderby" ~> "="~> propertyList ^^ OrderByAsc
  )

  lazy val select: PackratParser[Expression] =
    "$select"~>"="~> propertyList ^^ Select