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postulate _==_ : {A : Set} -> A -> A -> Set
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refl : {A : Set}{x : A} -> x == x
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{-# BUILTIN EQUAL _==_ #-}
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{-# BUILTIN REFL refl #-}
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primEqElim : {A : Set}(x : A)(C : (y : A) -> x == y -> Set) ->
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C x refl -> (y : A) -> (p : x == y) -> C y p
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elim-== = \{A : Set} -> primEqElim {A}
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subst : {A : Set}(C : A -> Set){x y : A} -> x == y -> C y -> C x
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subst C {x}{y} p Cy = elim-== x (\z _ -> C z -> C x) (\Cx -> Cx) y p Cy
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sym : {A : Set}{x y : A} -> x == y -> y == x
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sym {x = x}{y = y} p = subst (\z -> y == z) p refl
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trans : {A : Set}{x y z : A} -> x == y -> y == z -> x == z
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trans {x = x}{y = y}{z = z} xy yz = subst (\w -> w == z) xy yz
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cong : {A B : Set}{x y : A}(f : A -> B) -> x == y -> f x == f y
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cong {y = y} f xy = subst (\ ∙ -> f ∙ == f y) xy refl