add the %chance combinator

extra/combinators/lib/lib-docs.factor

@@ -11,3 +11,12 @@ HELP: generate
     "[ 20 random-prime ] [ 4 mod 3 = ] generate ."
     "526367"
 } ;
+
+HELP: %chance
+{ $values { "quot" quotation } { "n" integer } }
+{ $description "Calls the quotation " { $snippet "n" } " percent of the time." }
+{ $unchecked-example
+    "USING: io ;"
+    "[ \"hello, world!  maybe.\" print ] 50 %chance"
+    ""
+} ;

extra/combinators/lib/lib.factor

@@ -4,7 +4,7 @@
 ! See http://factorcode.org/license.txt for BSD license.
 USING: kernel combinators fry namespaces quotations hashtables
 sequences assocs arrays inference effects math math.ranges
-generalizations macros continuations locals ;
+generalizations macros continuations random locals ;
 
 IN: combinators.lib
 
@@ -147,3 +147,5 @@ MACRO: predicates ( seq -- quot/f )
     dup [ 1quotation [ drop ] prepend ] map
     >r [ [ dup ] prepend ] map r> zip [ drop f ] suffix
     [ cond ] curry ;
+
+: %chance ( quot integer -- ) 100 random > swap when ; inline