fix bug in polymorphic types (when creating a tuple of two empty lists, there was only one type variable (the type of the list), whereas each list could have a different type, so there should be two type variables) + make type variables look nicer

structures/types.js

@@ -62,7 +62,7 @@ export function prettyT(type) {
   }
   if (type.typeVars) {
     if (type.typeVars.size > 0) {
-      return `∀(${[...type.typeVars].map(prettyT).join(", ")}): ${prettyT(type.type)}`;
+      return `∀${[...type.typeVars].map(prettyT).sort((a,b)=>a.localeCompare(b)).join(",")}: ${prettyT(type.type)}`;
     }
     else {
       return prettyT(type.type);