Allow variable defs to use earlier names in list

Interpreter.cs

@@ -490,7 +490,9 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 			switch (binding)
 			{
 				case VarBinding(var pattern, var valueExpr):
-					var value = evaluate(env, valueExpr);
+					// By passing newEnv, we let the var definition refer to
+					// earlier bindings in the list.
+					var value = evaluate(newEnv, valueExpr);
 					try
 					{
 						pattern.accept((value, newEnv), new PatternBinder());