class Spruz::Limited

def execute

Execute _maximum_ number of threads in parallel. 
def execute
  @mutex.synchronize do
    loop do
      if @count < @maximum
        @count += 1
        Thread.new do
          yield
          @mutex.synchronize { @count -= 1 }
          @continue.signal
        end
        return
      else
        @continue.wait(@mutex)
      end
    end
  end
end

def initialize(maximum)

Create a Limited instance, that runs _maximum_ threads at most. 
def initialize(maximum)
  @mutex =  Mutex.new
  @continue = ConditionVariable.new
  @maximum = Integer(maximum)
  raise ArgumentError, "maximum < 1" if @maximum < 1
  @count = 0
end