class Concurrent::Promise

def then(rescuer = nil, &block)
  raise ArgumentError.new('rescuers and block are both missing') if rescuer.nil? && !block_given?
  block = Proc.new { |result| result } if block.nil?
  child = Promise.new(
    parent: self,
    executor: @executor,
    on_fulfill: block,
    on_reject: rescuer
  )
  mutex.synchronize do
    child.state = :pending if @state == :pending
    child.on_fulfill(apply_deref_options(@value)) if @state == :fulfilled
    child.on_reject(@reason) if @state == :rejected
    @children << child
  end
  child
end