module ActiveSupport::Concern

def included(base = nil, &block)

When you define more than one +included+ block, it raises an exception.
so that you can write class macros here.
Evaluate given block in context of base class, 
def included(base = nil, &block)
  if base.nil?
    if instance_variable_defined?(:@_included_block)
      if @_included_block.source_location != block.source_location
        raise MultipleIncludedBlocks
      end
    else
      @_included_block = block
    end
  else
    super
  end
end