--- a/progs/fsharp/re.ml	Fri Mar 18 15:03:54 2016 +0000
+++ b/progs/fsharp/re.ml	Sat Mar 19 23:27:29 2016 +0000
@@ -1,7 +1,7 @@
 
 type rexp =
-   NULL 
- | EMPTY 
+   ZERO 
+ | ONE 
  | CHAR of char
  | ALT of rexp * rexp
  | SEQ of rexp * rexp 
@@ -9,7 +9,7 @@
  | RECD of string * rexp;;
 
 type value =
-   Void
+   Empty
  | Chr of char
  | Sequ of value * value
  | Left of value
@@ -24,7 +24,7 @@
 
 (* some helper functions for rexps *)
 let rec seq s = match s with
-  | [] -> EMPTY
+  | [] -> ONE
   | [c] -> CHAR(c)
   | c::cs -> SEQ(CHAR(c), seq cs);;
 
@@ -41,15 +41,15 @@
 let ($) x r = RECD(x, r);;
 
 let alts rs = match rs with 
-  | [] -> NULL
+  | [] -> ZERO
   | [r] -> r
   | r::rs -> List.fold (++) r rs;;
 
 
 (* size of a regular expressions - for testing purposes *)
 let rec size r = match r with
-  | NULL -> 1
-  | EMPTY -> 1
+  | ZERO -> 1
+  | ONE -> 1
   | CHAR(_) -> 1
   | ALT(r1, r2) -> 1 + (size r1) + (size r2)
   | SEQ(r1, r2) -> 1 + (size r1) + (size r2)
@@ -59,8 +59,8 @@
 (* nullable function: tests whether the regular 
    expression can recognise the empty string *)
 let rec nullable r = match r with
-  | NULL -> false
-  | EMPTY -> true
+  | ZERO -> false
+  | ONE -> true
   | CHAR(_) -> false
   | ALT(r1, r2) -> nullable(r1) || nullable(r2)
   | SEQ(r1, r2) -> nullable(r1) && nullable(r2)
@@ -69,9 +69,9 @@
 
 (* derivative of a regular expression r w.r.t. a character c *)
 let rec der c r = match r with 
-  | NULL -> NULL
-  | EMPTY -> NULL
-  | CHAR(d) -> if c = d then EMPTY else NULL
+  | ZERO -> ZERO
+  | ONE -> ZERO
+  | CHAR(d) -> if c = d then ONE else ZERO
   | ALT(r1, r2) -> ALT(der c r1, der c r2)
   | SEQ(r1, r2) -> 
       if nullable r1 then ALT(SEQ(der c r1, r2), der c r2)
@@ -86,7 +86,7 @@
 
 (* extracts a string from value *)
 let rec flatten v = match v with 
-  | Void -> ""
+  | Empty -> ""
   | Chr(c) -> System.Convert.ToString(c)
   | Left(v) -> flatten v
   | Right(v) -> flatten v
@@ -97,7 +97,7 @@
 
 (* extracts an environment from a value *)
 let rec env v = match v with
-  | Void -> []
+  | Empty -> []
   | Chr(c) -> []
   | Left(v) -> env v
   | Right(v) -> env v
@@ -111,7 +111,7 @@
 
 (* the value for a nullable rexp *)
 let rec mkeps r = match r with
-  | EMPTY -> Void
+  | ONE -> Empty
   | ALT(r1, r2) -> 
       if nullable r1 then Left(mkeps r1) else Right(mkeps r2)
   | SEQ(r1, r2) -> Sequ(mkeps r1, mkeps r2)
@@ -127,7 +127,7 @@
   | SEQ(r1, r2), Right(v2) -> Sequ(mkeps r1, inj r2 c v2)
   | ALT(r1, r2), Left(v1) -> Left(inj r1 c v1)
   | ALT(r1, r2), Right(v2) -> Right(inj r2 c v2)
-  | CHAR(d), Void -> Chr(d) 
+  | CHAR(d), Empty -> Chr(d) 
   | RECD(x, r1), _ -> Rec(x, inj r1 c v);;
 
 (* some "rectification" functions for simplification *)
@@ -139,8 +139,8 @@
   | Left(v) -> Left(f1 v);;
 let f_seq f1 f2 = fun v -> match v with 
   Sequ(v1, v2) -> Sequ(f1 v1, f2 v2);;
-let f_seq_Void1 f1 f2 = fun v -> Sequ(f1 Void, f2 v);;
-let f_seq_Void2 f1 f2 = fun v -> Sequ(f1 v, f2 Void);;
+let f_seq_Empty1 f1 f2 = fun v -> Sequ(f1 Empty, f2 v);;
+let f_seq_Empty2 f1 f2 = fun v -> Sequ(f1 v, f2 Empty);;
 let f_rec f = fun v -> match v with
     Rec(x, v) -> Rec(x, f v);;
 
@@ -151,18 +151,18 @@
       let (r1s, f1s) = simp r1 in 
       let (r2s, f2s) = simp r2 in
       (match r1s, r2s with
-          NULL, _ -> (r2s, f_right f2s)
-        | _, NULL -> (r1s, f_left f1s)
+          ZERO, _ -> (r2s, f_right f2s)
+        | _, ZERO -> (r1s, f_left f1s)
         | _, _    -> if r1s = r2s then (r1s, f_left f1s)
                      else (ALT (r1s, r2s), f_alt f1s f2s)) 
   | SEQ(r1, r2) -> 
       let (r1s, f1s) = simp r1 in
       let (r2s, f2s) = simp r2 in
       (match r1s, r2s with
-          NULL, _  -> (NULL, f_right f2s)
-        | _, NULL  -> (NULL, f_left f1s)
-        | EMPTY, _ -> (r2s, f_seq_Void1 f1s f2s)
-        | _, EMPTY -> (r1s, f_seq_Void2 f1s f2s)
+          ZERO, _  -> (ZERO, f_right f2s)
+        | _, ZERO  -> (ZERO, f_left f1s)
+        | ONE, _ -> (r2s, f_seq_Empty1 f1s f2s)
+        | _, ONE -> (r1s, f_seq_Empty2 f1s f2s)
         | _, _     -> (SEQ(r1s, r2s), f_seq f1s f2s))
   | RECD(x, r1) -> 
       let (r1s, f1s) = simp r1 in