class ActiveSupport::Dependencies::WatchStack

found by child.rb to the list of original constants in parent.rb.
This is handled by walking back up the watch stack and adding the constants
autoloaded_constants. If it was being required, they will be discarded.
If child.rb is being autoloaded, its constants will be added to
handles the new constants.
load of another file (child.rb) the stack will ensure that child.rb
loaded. If a file in the process of being loaded (parent.rb) triggers the
The WatchStack keeps a stack of the modules being watched as files are

def each(&block) 

def each(&block)
  @stack.each(&block)
end

def initialize 

def initialize
  @watching = []
  @stack = Hash.new { |h, k| h[k] = [] }
end

def new_constants

watch_namespaces.
Returns a list of new constants found since the last call to 
def new_constants
  constants = []
  # Grab the list of namespaces that we're looking for new constants under
  @watching.last.each do |namespace|
    # Retrieve the constants that were present under the namespace when watch_namespaces
    # was originally called
    original_constants = @stack[namespace].last
    mod = Inflector.constantize(namespace) if Dependencies.qualified_const_defined?(namespace)
    next unless mod.is_a?(Module)
    # Get a list of the constants that were added
    new_constants = mod.constants(false) - original_constants
    # @stack[namespace] returns an Array of the constants that are being evaluated
    # for that namespace. For instance, if parent.rb requires child.rb, the first
    # element of @stack[Object] will be an Array of the constants that were present
    # before parent.rb was required. The second element will be an Array of the
    # constants that were present before child.rb was required.
    @stack[namespace].each do |namespace_constants|
      namespace_constants.concat(new_constants)
    end
    # Normalize the list of new constants, and add them to the list we will return
    new_constants.each do |suffix|
      constants << ([namespace, suffix] - ["Object"]).join("::")
    end
  end
  constants
ensure
  # A call to new_constants is always called after a call to watch_namespaces
  pop_modules(@watching.pop)
end

def pop_modules(modules) 

def pop_modules(modules)
  modules.each { |mod| @stack[mod].pop }
end

def watch_namespaces(namespaces)

constants.
Add a set of modules to the watch stack, remembering the initial 
def watch_namespaces(namespaces)
  @watching << namespaces.map do |namespace|
    module_name = Dependencies.to_constant_name(namespace)
    original_constants = Dependencies.qualified_const_defined?(module_name) ?
      Inflector.constantize(module_name).constants(false) : []
    @stack[module_name] << original_constants
    module_name
  end
end

def watching? 

def watching?
  !@watching.empty?
end