make demo reproducible with srand

demos/MyModelArgs.rb

@@ -8,15 +8,16 @@ class MyModel
 
   def init
     @x = 1
-    schedule(:increment, rand, 1, 97)
+    schedule(:increment, rand(2), 1, 97)
   end
 
   def increment(n, c)
     @x += n
-    schedule(:increment, 2.0 * rand, @x, c + 1)
+    schedule(:increment, 2.0 * rand(2), @x, c + 1)
     printf "%f, %f, %c\n", model_time, @x, c
     schedule(:halt, 0.0) if model_time > 10
   end
 end
 
+srand(42)
 MyModel.new.run