AS guide: documents Enumerable#sum

railties/guides/source/active_support_overview.textile

@@ -802,6 +802,51 @@ end
 
 WARNING. Active Support redefines +group_by+ in Ruby 1.8.7 so that it still returns an ordered hash.
 
+h4. +sum+
+
+The method +sum+ adds the elements of an enumerable:
+
+<ruby>
+[1, 2, 3].sum # => 6
+(1..100).sum  # => 5050
+</ruby>
+
+Addition only assumes the elements respond to <tt>+</tt>:
+
+<ruby>
+[[1, 2], [2, 3], [3, 4]].sum    # => [1, 2, 2, 3, 3, 4]
+%w(foo bar baz).sum             # => "foobarbaz"
+{:a => 1, :b => 2, :c => 3}.sum # => [:b, 2, :c, 3, :a, 1]
+</ruby>
+
+The sum of an empty collection is zero by default, but this is customizable:
+
+<ruby>
+[].sum    # => 0
+[].sum(1) # => 1
+</ruby>
+
+If a block is given +sum+ becomes an iterator that yields the elements of the collection and sums the returned values:
+
+<ruby>
+(1..5).sum {|n| n * 2 } # => 30
+[2, 4, 6, 8, 10].sum    # => 30
+</ruby>
+
+The sum of an empty receiver can be customized in this form as well:
+
+<ruby>
+[].sum(1) {|n| n**3} # => 1
+</ruby>
+
+The method +ActiveRecord::Observer#observed_subclasses+ for example is implemented this way:
+
+<ruby>
+def observed_subclasses
+  observed_classes.sum([]) { |klass| klass.send(:subclasses) }
+end
+</ruby>
+
 h3. Extensions to +Array+
 
 h4. Accessing