def output(place, &block)
      raise "This is not a Place object!" unless place.kind_of? Place
      raise "Tried to define output arc without expression! Block is required!" unless block_given?
      @outputs << OutputArc.new(place, block)
    end
def output(place, &block)
      raise "This is not a Place object!" unless place.kind_of? Place
      raise "Tried to define output arc without expression! Block is required!" unless block_given?
      @outputs << OutputArc.new(place, block)
    end
Add output arc to the +place+.
 +block+ is the arc's expresstion, it will be called while firing
 transition. Value returned from the block will be put in output
 place. The block gets +binding+, and +clock+ values. +binding+ is
 a hash with names of input places as keys nad tokens as values.