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(* Simple let-patterns *)
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Definition l1 (t : A * B * B) : A := let '(x, y, z) := t in x.
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Definition l2 (t : (A * B) * B) : A := let '((x, y), z) := t in x.
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Definition l3 (t : A * (B * B)) : A := let '(x, (y, z)) := t in x.
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Record someT (A : Type) := mkT { a : nat; b: A }.
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Definition l4 A (t : someT A) : nat := let 'mkT x y := t in x.
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Definition l5 A (B : A -> Type) (t : sigT B) : B (projT1 t) :=
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let 'existT x y := t return B (projT1 t) in y.
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Definition l6 A (B : A -> Type) (t : sigT B) : B (projT1 t) :=
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let 'existT x y as t' := t return B (projT1 t') in y.
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Definition l7 A (B : A -> Type) (t : sigT B) : B (projT1 t) :=
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let 'existT x y as t' in sigT _ := t return B (projT1 t') in y.
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Definition l8 A (B : A -> Type) (t : sigT B) : B (projT1 t) :=
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(** An example from algebra, using let' and inference of return clauses
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to deconstruct contexts. *)
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Record a_category (A : Type) (hom : A -> A -> Type) := { }.
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Definition category := { A : Type & { hom : A -> A -> Type & a_category A hom } }.
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Record a_functor (A : Type) (hom : A -> A -> Type) (C : a_category A hom) := { }.
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Notation " x :& y " := (@existT _ _ x y) (right associativity, at level 55) : core_scope.
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Definition functor (c d : category) :=
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let ' A :& homA :& CA := c in
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let ' B :& homB :& CB := d in
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Definition identity_functor (c : category) : functor c c :=
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let 'A :& homA :& CA := c in
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Definition functor_composition (a b c : category) : functor a b -> functor b c -> functor a c :=
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let 'A :& homA :& CA := a in
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let 'B :& homB :& CB := b in
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let 'C :& homB :& CB := c in