demo: add a fun deck-shuffling example

Discovered this today. It uses the newly implemented binary ? operator,
"deal".

demo/demo.ivy

@@ -235,5 +235,14 @@ op primes N = (not T in T o.* T) sel T = 1 drop iota N
 # The assignment to T gives 2..N. We use outer product to build an array of all products.
 # Then we find all elements of T that appear in the product matrix, invert that, and select from the original.
 primes 100
+# A final trick.
+# The binary ? operator "deals": x?y selects at random x distinct integers from 1..y inclusive.
+5?10
+# We can use this to shuffle a deck of cards. The suits are ♠♡♣♢, the values
+# A234567890JQK (using 0 for 10, for simplicity).
+# Create the deck using outer product with the ravel operator:
+"A234567890JQK" o., "♠♡♣♢"
+# To shuffle it, ravel into into a vector and index that by 1 through 52, shuffled.
+(, "A234567890JQK" o., "♠♡♣♢")[52?52]
 # That's it! Have fun.
 # For more information visit https://godoc.org/robpike.io/ivy

demo/demo.out

@@ -291,3 +291,18 @@ wroo
 46
 5
 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97
+7 4 9 5 2
+A♠ A♡ A♣ A♢
+2♠ 2♡ 2♣ 2♢
+3♠ 3♡ 3♣ 3♢
+4♠ 4♡ 4♣ 4♢
+5♠ 5♡ 5♣ 5♢
+6♠ 6♡ 6♣ 6♢
+7♠ 7♡ 7♣ 7♢
+8♠ 8♡ 8♣ 8♢
+9♠ 9♡ 9♣ 9♢
+0♠ 0♡ 0♣ 0♢
+J♠ J♡ J♣ J♢
+Q♠ Q♡ Q♣ Q♢
+K♠ K♡ K♣ K♢
+J♡ Q♠ Q♢ 8♡ K♢ 4♠ 6♡ 8♣ 7♠ 8♢ 2♢ 4♡ 4♢ 0♣ A♠ 9♣ 7♣ 7♡ 5♣ Q♣ 3♣ 5♢ 2♡ J♣ J♢ 0♠ A♢ 9♡ J♠ 6♢ 9♠ K♣ 5♡ K♠ 3♡ 0♢ 9♢ 7♢ 6♠ A♡ 5♠ 3♢ A♣ 6♣ Q♡ 2♣ K♡ 2♠ 8♠ 3♠ 4♣ 0♡