arithm example

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diff --git a/examples/logic/ArithmEng.gf b/examples/logic/ArithmEng.gf
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+--# -path=.:mathematical:present:resource-1.0/api:prelude
+
+concrete ArithmEng of Arithm = LogicEng ** 
+  open
+    GrammarEng,
+    ParadigmsEng,
+    ProoftextEng,
+    MathematicalEng,
+    CombinatorsEng,
+    ConstructorsEng
+  in { 
+
+lin
+  Nat  = UseN  (regN "number") ;
+  Zero = UsePN (regPN "zero") ;
+  Succ = appN2 (regN2 "successor") ;
+
+  EqNat x y = mkS (predA2 (mkA2 (regA "equal") (mkPrep "to")) x y) ;
+--  LtNat = adj2 ["smaller than"] ;
+--  Div   = adj2 ["divisible by"] ;
+  Even  x = mkS (predA (regA "even") x) ;
+  Odd   x = mkS (predA (regA "odd") x) ;
+  Prime x = mkS (predA (regA "prime") x) ;
+
+  one = UsePN (regPN "one") ;
+  two = UsePN (regPN "two") ;
+  sum = appColl (regN2 "sum") ;
+  prod = appColl (regN2 "product") ;
+
+  evax1 = 
+    proof (by (ref (mkLabel ["the first axiom of evenness ,"]))) 
+      (mkS (pred (regA "even") (UsePN (regPN "zero")))) ;
+  evax2 n c = 
+    appendText c 
+      (proof (by (ref (mkLabel ["the second axiom of evenness ,"]))) 
+      (mkS (pred (regA "odd") (appN2 (regN2 "successor") (UsePN (regPN "zero")))))) ;
+  evax3 n c = 
+    appendText c 
+      (proof (by (ref (mkLabel ["the third axiom of evenness ,"]))) 
+      (mkS (pred (regA "even") (appN2 (regN2 "successor") (UsePN (regPN "zero")))))) ;
+
+{-
+
+  eqax1 = ss ["by the first axiom of equality , zero is equal to zero"] ;
+  eqax2 m n c = {s = 
+    c.s ++ ["by the second axiom of equality , the successor of"] ++ m.s ++ 
+    ["is equal to the successor of"] ++ n.s} ;
+-}
+
+  IndNat C d e = {s = 
+    ["we proceed by induction . for the basis ,"] ++ d.s ++ 
+    ["for the induction step, consider a number"] ++ C.$0 ++ 
+    ["and assume"] ++ C.s ++ 
+    --- "(" ++ e.$1 ++ ")" ++ 
+    "." ++ e.s ++ 
+    ["hence , for all numbers"] ++ C.$0 ++ "," ++ C.s ; lock_Text = <>} ;
+
+  ex1 = proof ["the first theorem and its proof"] ;
+
+} ;
+