[ OT ] What about rights/authorship ?

A mod might be able to come in here and offer better advice but my understanding is that an *idea* can not be copyrighted and so can not be stolen. 
 
 That seems to be backed up by some searches I did and found info [ here ] ( http: //www.copyrightservice.co.uk/services/knowledge-base/kb_idea ), [ here ] ( https: //www.plagiarismtoday.com/2010/01/08/5-things-that-cant-be-copyrighted/ ) and a bunch of other places. 
 
 A prompt is an idea, it's the story behind it that would be protected and so I think taking inspiration for your game is fine. There would be no need to make a payment, although I am sure that the original poster would be grateful for some sort of credit or acknowledgement. 
 
 ***** 
 
 EDIT: /u/SurvivorType beat me to it!
