(.module:
  [library
   [lux {"-" [Type]}
    [abstract
     [equivalence {"+" [Equivalence]}]
     [order {"+" [Order]}]
     [hash {"+" [Hash]}]]
    [data
     ["." product]
     ["." text ("#\." order)
      ["%" format {"+" [format]}]]]]]
  ["." // "_"
   ["#" artifact {"+" [Artifact]} ("#\." order)
    [type {"+" [Type]}]]])

(type: .public Dependency
  (Record
   [#artifact Artifact
    #type Type]))

(def: .public hash
  (Hash Dependency)
  ($_ product.hash
      //.hash
      text.hash
      ))

(def: .public equivalence
  (Equivalence Dependency)
  (\ hash &equivalence))

(implementation: .public order
  (Order Dependency)

  (def: &equivalence
    ..equivalence)

  (def: (< reference subject)
    (<| (or (//\< (value@ #artifact reference)
                  (value@ #artifact subject)))
        
        (and (//\= (value@ #artifact reference)
                   (value@ #artifact subject)))
        (text\< (value@ #type reference)
                (value@ #type subject)))))