code: function( count ) {
  		var test = this.test,
  			popped = false,
  			acceptCallCount = count;
  
  		if ( typeof acceptCallCount === "undefined" ) {
  			acceptCallCount = 1;
  		}
  
  		test.semaphore += 1;
  		test.usedAsync = true;
  		pauseProcessing();
  
  		return function done() {
  
  			if ( popped ) {
  				test.pushFailure( "Too many calls to the `assert.async` callback",
  					sourceFromStacktrace( 2 ) );
  				return;
  			}
  			acceptCallCount -= 1;
  			if ( acceptCallCount > 0 ) {
  				return;
  			}
  
  			test.semaphore -= 1;
  			popped = true;
  			resumeProcessing();
  		};
  	}
docs: Increment this Test's semaphore counter, then return a function that decrements that counter a maximum of once.