src/CTT/CTT.thy
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(*  Title:      CTT/ctt.thy
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    ID:         $Id$
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    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
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    Copyright   1993  University of Cambridge
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Constructive Type Theory
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*)
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CTT = Pure +
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types i,t,o 0
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arities i,t,o :: logic
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consts
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  (*Types*)
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  F,T       :: "t"          (*F is empty, T contains one element*)
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  contr     :: "i=>i"
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  tt        :: "i"
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  (*Natural numbers*)
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  N         :: "t"
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  succ      :: "i=>i"
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  rec       :: "[i, i, [i,i]=>i] => i"
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  (*Unions*)
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  inl,inr   :: "i=>i"
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  when      :: "[i, i=>i, i=>i]=>i"
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  (*General Sum and Binary Product*)
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  Sum       :: "[t, i=>t]=>t"
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  fst,snd   :: "i=>i"
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  split     :: "[i, [i,i]=>i] =>i"
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  (*General Product and Function Space*)
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  Prod      :: "[t, i=>t]=>t"
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  (*Equality type*)
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  Eq        :: "[t,i,i]=>t"
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  eq        :: "i"
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  (*Judgements*)
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  Type      :: "t => prop"          ("(_ type)" [10] 5)
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  Eqtype    :: "[t,t]=>prop"        ("(3_ =/ _)" [10,10] 5)
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  Elem      :: "[i, t]=>prop"       ("(_ /: _)" [10,10] 5)
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  Eqelem    :: "[i,i,t]=>prop"      ("(3_ =/ _ :/ _)" [10,10,10] 5)
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  Reduce    :: "[i,i]=>prop"        ("Reduce[_,_]")
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  (*Types*)
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  "@PROD"   :: "[idt,t,t]=>t"       ("(3PROD _:_./ _)" 10)
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  "@SUM"    :: "[idt,t,t]=>t"       ("(3SUM _:_./ _)" 10)
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  "+"       :: "[t,t]=>t"           (infixr 40)
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  (*Invisible infixes!*)
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  "@-->"    :: "[t,t]=>t"           ("(_ -->/ _)" [31,30] 30)
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  "@*"      :: "[t,t]=>t"           ("(_ */ _)" [51,50] 50)
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  (*Functions*)
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  lambda    :: "(i => i) => i"      (binder "lam " 10)
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  "`"       :: "[i,i]=>i"           (infixl 60)
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  (*Natural numbers*)
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  "0"       :: "i"                  ("0")
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  (*Pairing*)
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  pair      :: "[i,i]=>i"           ("(1<_,/_>)")
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translations
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  "PROD x:A. B" => "Prod(A, %x. B)"
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  "A --> B"     => "Prod(A, _K(B))"
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  "SUM x:A. B"  => "Sum(A, %x. B)"
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  "A * B"       => "Sum(A, _K(B))"
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rules
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  (*Reduction: a weaker notion than equality;  a hack for simplification.
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    Reduce[a,b] means either that  a=b:A  for some A or else that "a" and "b"
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    are textually identical.*)
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  (*does not verify a:A!  Sound because only trans_red uses a Reduce premise
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    No new theorems can be proved about the standard judgements.*)
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  refl_red "Reduce[a,a]"
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  red_if_equal "a = b : A ==> Reduce[a,b]"
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  trans_red "[| a = b : A;  Reduce[b,c] |] ==> a = c : A"
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  (*Reflexivity*)
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  refl_type "A type ==> A = A"
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  refl_elem "a : A ==> a = a : A"
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  (*Symmetry*)
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  sym_type  "A = B ==> B = A"
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  sym_elem  "a = b : A ==> b = a : A"
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  (*Transitivity*)
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  trans_type   "[| A = B;  B = C |] ==> A = C"
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  trans_elem   "[| a = b : A;  b = c : A |] ==> a = c : A"
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  equal_types  "[| a : A;  A = B |] ==> a : B"
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  equal_typesL "[| a = b : A;  A = B |] ==> a = b : B"
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  (*Substitution*)
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  subst_type   "[| a : A;  !!z. z:A ==> B(z) type |] ==> B(a) type"
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  subst_typeL  "[| a = c : A;  !!z. z:A ==> B(z) = D(z) |] ==> B(a) = D(c)"
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  subst_elem   "[| a : A;  !!z. z:A ==> b(z):B(z) |] ==> b(a):B(a)"
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  subst_elemL
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    "[| a=c : A;  !!z. z:A ==> b(z)=d(z) : B(z) |] ==> b(a)=d(c) : B(a)"
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  (*The type N -- natural numbers*)
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  NF "N type"
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  NI0 "0 : N"
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  NI_succ "a : N ==> succ(a) : N"
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  NI_succL  "a = b : N ==> succ(a) = succ(b) : N"
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  NE
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   "[| p: N;  a: C(0);  !!u v. [| u: N; v: C(u) |] ==> b(u,v): C(succ(u)) |] \
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\   ==> rec(p, a, %u v.b(u,v)) : C(p)"
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  NEL
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   "[| p = q : N;  a = c : C(0);  \
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\      !!u v. [| u: N; v: C(u) |] ==> b(u,v) = d(u,v): C(succ(u)) |] \
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\   ==> rec(p, a, %u v.b(u,v)) = rec(q,c,d) : C(p)"
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  NC0
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   "[| a: C(0);  !!u v. [| u: N; v: C(u) |] ==> b(u,v): C(succ(u)) |] \
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\   ==> rec(0, a, %u v.b(u,v)) = a : C(0)"
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  NC_succ
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   "[| p: N;  a: C(0);  \
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\       !!u v. [| u: N; v: C(u) |] ==> b(u,v): C(succ(u)) |] ==>  \
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\   rec(succ(p), a, %u v.b(u,v)) = b(p, rec(p, a, %u v.b(u,v))) : C(succ(p))"
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  (*The fourth Peano axiom.  See page 91 of Martin-Lof's book*)
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  zero_ne_succ
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    "[| a: N;  0 = succ(a) : N |] ==> 0: F"
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  (*The Product of a family of types*)
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  ProdF  "[| A type; !!x. x:A ==> B(x) type |] ==> PROD x:A.B(x) type"
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  ProdFL
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   "[| A = C;  !!x. x:A ==> B(x) = D(x) |] ==> \
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\   PROD x:A.B(x) = PROD x:C.D(x)"
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  ProdI
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   "[| A type;  !!x. x:A ==> b(x):B(x)|] ==> lam x.b(x) : PROD x:A.B(x)"
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  ProdIL
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   "[| A type;  !!x. x:A ==> b(x) = c(x) : B(x)|] ==> \
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\   lam x.b(x) = lam x.c(x) : PROD x:A.B(x)"
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  ProdE  "[| p : PROD x:A.B(x);  a : A |] ==> p`a : B(a)"
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  ProdEL "[| p=q: PROD x:A.B(x);  a=b : A |] ==> p`a = q`b : B(a)"
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  ProdC
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   "[| a : A;  !!x. x:A ==> b(x) : B(x)|] ==> \
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\   (lam x.b(x)) ` a = b(a) : B(a)"
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  ProdC2
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   "p : PROD x:A.B(x) ==> (lam x. p`x) = p : PROD x:A.B(x)"
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  (*The Sum of a family of types*)
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  SumF  "[| A type;  !!x. x:A ==> B(x) type |] ==> SUM x:A.B(x) type"
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  SumFL
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    "[| A = C;  !!x. x:A ==> B(x) = D(x) |] ==> SUM x:A.B(x) = SUM x:C.D(x)"
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  SumI  "[| a : A;  b : B(a) |] ==> <a,b> : SUM x:A.B(x)"
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  SumIL "[| a=c:A;  b=d:B(a) |] ==> <a,b> = <c,d> : SUM x:A.B(x)"
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  SumE
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    "[| p: SUM x:A.B(x);  !!x y. [| x:A; y:B(x) |] ==> c(x,y): C(<x,y>) |] \
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\    ==> split(p, %x y.c(x,y)) : C(p)"
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  SumEL
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    "[| p=q : SUM x:A.B(x); \
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\       !!x y. [| x:A; y:B(x) |] ==> c(x,y)=d(x,y): C(<x,y>)|] \
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\    ==> split(p, %x y.c(x,y)) = split(q, % x y.d(x,y)) : C(p)"
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  SumC
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    "[| a: A;  b: B(a);  !!x y. [| x:A; y:B(x) |] ==> c(x,y): C(<x,y>) |] \
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\    ==> split(<a,b>, %x y.c(x,y)) = c(a,b) : C(<a,b>)"
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  fst_def   "fst(a) == split(a, %x y.x)"
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  snd_def   "snd(a) == split(a, %x y.y)"
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  (*The sum of two types*)
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  PlusF   "[| A type;  B type |] ==> A+B type"
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  PlusFL  "[| A = C;  B = D |] ==> A+B = C+D"
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  PlusI_inl   "[| a : A;  B type |] ==> inl(a) : A+B"
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  PlusI_inlL "[| a = c : A;  B type |] ==> inl(a) = inl(c) : A+B"
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  PlusI_inr   "[| A type;  b : B |] ==> inr(b) : A+B"
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  PlusI_inrL "[| A type;  b = d : B |] ==> inr(b) = inr(d) : A+B"
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  PlusE
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    "[| p: A+B;  !!x. x:A ==> c(x): C(inl(x));  \
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\                !!y. y:B ==> d(y): C(inr(y)) |] \
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\    ==> when(p, %x.c(x), %y.d(y)) : C(p)"
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  PlusEL
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    "[| p = q : A+B;  !!x. x: A ==> c(x) = e(x) : C(inl(x));   \
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\                     !!y. y: B ==> d(y) = f(y) : C(inr(y)) |] \
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\    ==> when(p, %x.c(x), %y.d(y)) = when(q, %x.e(x), %y.f(y)) : C(p)"
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  PlusC_inl
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    "[| a: A;  !!x. x:A ==> c(x): C(inl(x));  \
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\              !!y. y:B ==> d(y): C(inr(y)) |] \
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\    ==> when(inl(a), %x.c(x), %y.d(y)) = c(a) : C(inl(a))"
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  PlusC_inr
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    "[| b: B;  !!x. x:A ==> c(x): C(inl(x));  \
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\              !!y. y:B ==> d(y): C(inr(y)) |] \
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\    ==> when(inr(b), %x.c(x), %y.d(y)) = d(b) : C(inr(b))"
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  (*The type Eq*)
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  EqF    "[| A type;  a : A;  b : A |] ==> Eq(A,a,b) type"
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  EqFL "[| A=B;  a=c: A;  b=d : A |] ==> Eq(A,a,b) = Eq(B,c,d)"
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  EqI "a = b : A ==> eq : Eq(A,a,b)"
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  EqE "p : Eq(A,a,b) ==> a = b : A"
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  (*By equality of types, can prove C(p) from C(eq), an elimination rule*)
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  EqC "p : Eq(A,a,b) ==> p = eq : Eq(A,a,b)"
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  (*The type F*)
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  FF "F type"
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  FE "[| p: F;  C type |] ==> contr(p) : C"
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  FEL  "[| p = q : F;  C type |] ==> contr(p) = contr(q) : C"
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  (*The type T
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     Martin-Lof's book (page 68) discusses elimination and computation.
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     Elimination can be derived by computation and equality of types,
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     but with an extra premise C(x) type x:T.
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     Also computation can be derived from elimination. *)
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  TF "T type"
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  TI "tt : T"
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  TE "[| p : T;  c : C(tt) |] ==> c : C(p)"
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  TEL "[| p = q : T;  c = d : C(tt) |] ==> c = d : C(p)"
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  TC "p : T ==> p = tt : T"
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end
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ML
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val print_translation =
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  [("Prod", dependent_tr' ("@PROD", "@-->")),
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   ("Sum", dependent_tr' ("@SUM", "@*"))];
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