def evaluate_step_proc(step_proc)
      result = instance_exec(&step_proc)
      return result if result.is_a? Integer
      fail 'step Proc does not evaluate to an Integer'
    end
def evaluate_step_proc(step_proc)
      result = instance_exec(&step_proc)
      return result if result.is_a? Integer
      fail 'step Proc does not evaluate to an Integer'
    end
Executes a proc and returns its +Integer+ value

 @param [ Proc ] step_proc The +Proc+ to call

 @raise [ ArgumentError ] When +step_proc+ does not evaluate to +Integer+

 @return [ Integer ] The number to add to the current increment