2013-08-16: define the length of the hash-table

define the length of the hash-table

diff --git a/unicode-math-fun.w b/unicode-math-fun.w
--- a/unicode-math-fun.w
+++ b/unicode-math-fun.w
@@ -10,7 +10,7 @@ define : ∪ . lists
     apply append lists
 
 define : ∩ list1 list2
-    let : : h : make-hash-table
+    let : : h : make-hash-table : length list2
         let fill : : toadd list2
             when : not : equal? toadd '()
                 hash-set! h (list-ref toadd 0) #t