2017-07-24: add multiplication

add multiplication

diff --git a/examples/unicode-math-fun.w b/examples/unicode-math-fun.w
--- a/examples/unicode-math-fun.w
+++ b/examples/unicode-math-fun.w
@@ -9,6 +9,9 @@ exec guile -L $(dirname $(dirname $(real
 define : Σ . n
     apply + n
 
+define : Π . n
+    apply * n
+
 define : ∪ . lists
     apply append lists
 
@@ -30,6 +33,8 @@ define : ∩ list1 list2
 
 display : Σ 1 2 8 0  5 7 59 12 5
 newline
+display : Π 1 2 8 0  5 7 59 12 5
+newline
 display : ∪ '(1 2 3) '(4 5 6)
 newline
 display : ∩ '(1 789 7 897 89 78 78 97 89 2 3 6) '(4 5 6 2  8 7 879 879 879 879 8797 97 97 987 89789 7 7897 987 897 987 87 897 896)