module ActiveSupport::Concern

def prepended(base = nil, &block)

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