improved example rightmostbittrick: one assert less, 64 bit

examples/rightmostbittrick.mlw

 module Rmbt
   use import int.Int
   use import ref.Ref
-  use import bv.BV32
+  use import bv.BV64
 
   let ghost rightmost_position_set (a : t) : t
     requires { a <> zero }
-    ensures  { ult result (of_int 32) }
+    ensures  { ult result (of_int 64) }
     ensures  { eq_sub_bv a zero zero result }
     ensures  { nth_bv a result }
   =
     let i = ref zero in
-    while ult !i (of_int 32) && not (nth_bv a !i) do
-      variant {32 - to_uint !i}
+    while ult !i (of_int 64) && not (nth_bv a !i) do
+      variant {64 - to_uint !i}
       invariant {eq_sub_bv a zero zero !i}
       i := add !i (of_int 1)
     done;
@@ -20,17 +20,16 @@ module Rmbt
   let rightmost_bit_trick (x: t) (ghost p : ref int) : t
     requires { x <> zero }
     writes   { p }
-    ensures  { 0 <= !p < 32 }
+    ensures  { 0 <= !p < 64 }
     ensures  { eq_sub x zero 0 !p }
     ensures  { nth x !p }
     ensures  { eq_sub result zero 0 !p }
-    ensures  { eq_sub result zero (!p+1) 32 }
+    ensures  { eq_sub result zero (!p+1) (63 - !p) }
     ensures  { nth result !p }
   =
     let ghost p_bv = rightmost_position_set x in
     ghost p := to_uint p_bv;
     assert { nth_bv (neg x) p_bv };
-    let res = bw_and x (neg x) in
-    assert {eq_sub_bv res zero (add p_bv (of_int 1)) (sub (of_int 31) p_bv )};
-    res
+    bw_and x (neg x)
+
 end
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