Adds reduce, refs #1731

Author: JJ

doc/Type/Any.pod6

@@ -900,7 +900,7 @@ function.
 my @years = (2003..2008).map( { Date.new( $_~"-01-01" ) } );
 @years.classify( *.is-leap-year , into => my %leap-years );
 say %leap-years;
-# OUTPUT: «{False => [2003-01-01 2005-01-01 2006-01-01 2007-01-01], 
+# OUTPUT: «{False => [2003-01-01 2005-01-01 2006-01-01 2007-01-01],
 #           True => [2004-01-01 2008-01-01]}␤»
 
 
@@ -909,6 +909,22 @@ normalized by the C<Callable> passed with the C<:as> argument, and it can use
 the C<:into> named argument to pass a C<Hash> the results will be classified
 into; in the example above, it's defined on the fly.
 
+=head2 method reduce
+
+Defined as:
+
+    multi method reduce(Any:U: & --> Nil)
+    multi method reduce(Any:D: &with)
+    multi sub reduce (&with, +list)
+
+Applying it to a class will always produce Nil.
+Applies its argument (or first argument, in case it's a sub) as an operator to
+all the elements in the object (or second argument), producing a single result.
+The argument must be an infix operator or take, in any case, two positional
+arguments.
+
+    (1..13).reduce( &[*] ).say; # OUTPUT: «6227020800␤»
+
 =head2 method produce
 
 Defined as: