class Concurrent::Collection::NonConcurrentMapBackend

def initialize(options = nil, &default_proc)

reasons.
SynchronizedMapBackend which uses a non-reentrant mutex for performance
directly without calling each other. This is important because of the
WARNING: all public methods of the class must operate on the @backend 
def initialize(options = nil, &default_proc)
  validate_options_hash!(options) if options.kind_of?(::Hash)
  set_backend(default_proc)
  @default_proc = default_proc
end