enum.rb

def m
  yield :a
end

def n
  yield :a, 1
end

m { |x| p x }
n { |x| p x }

m { |*x| p x }
n { |*x| p x }

p [:a, :b].to_enum.to_a
p [:a, :b].each_with_index.to_a
[:a, :b].each_with_index { |x| p x }

gistfile1.txt

sasha:rubinius2.0 brian$ ruby -v enum.rb 
ruby 1.8.7 (2009-06-12 patchlevel 174) [universal-darwin10.0]
:a
enum.rb:10: warning: multiple values for a block parameter (2 for 1)
	from enum.rb:6
[:a, 1]
[:a]
[:a, 1]
[:a, :b]
[[:a, 0], [:b, 1]]
enum.rb:17: warning: multiple values for a block parameter (2 for 1)
	from enum.rb:10
[:a, 0]
enum.rb:17: warning: multiple values for a block parameter (2 for 1)
	from enum.rb:10
[:b, 1]
sasha:rubinius2.0 brian$ rvm use 1.9.2
Using /Users/brian/.rvm/gems/ruby-1.9.2-p180
sasha:rubinius2.0 brian$ ruby -v enum.rb 
ruby 1.9.2p180 (2011-02-18 revision 30909) [x86_64-darwin10.7.0]
:a
:a
[:a]
[:a, 1]
[:a, :b]
[[:a, 0], [:b, 1]]
:a
:b