Improved loop for parameters of Bernoulli expneg greater than 1 (#157)

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under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).

src/DafnyVMCTrait.dfy

@@ -83,8 +83,12 @@ module DafnyVMCTrait {
         o := true;
       } else {
         var B := BernoulliExpNegSampleUnit(1, 1);
-        var R := BernoulliExpNegSampleGenLoop(iter - 1);
-        o := B == true && R == true;
+        if ! (B == true) {
+          o := B;
+        } else {
+          var R := BernoulliExpNegSampleGenLoop(iter - 1);
+          o := R;
+        }
       }
     }