code: function() {
  			var lastContext = this.forkContext;
  
  			this.forkContext = lastContext.upper;
  			this.forkContext.replaceHead(lastContext.makeNext(0, -1));
  
  			return lastContext;
  		}
docs: Pops and merges the last forking context.
  @returns {ForkContext} The last context.