title: How to make a schema for an unordered list where some occur once, some many times
body: <p>This is a similar question to <a href="https://stackoverflow.com/questions/3382944/how-to-create-a-schema-for-an-unordered-list-of-xml-nodes-with-occurance-constr">How to create a schema for an unordered list of XML nodes, with occurrence constraints</a>, but actually slightly simpler. However I am having great trouble understanding the logic behind sequences and choices (and especially when they are nested into sequences of choices or choices of sequences), and although I've studied it for a long time I can't understand how the example above works.</p>
  
  <p>What I need is schema for a list of nodes, ChildA and ChildB, whereby ChildA can occur 0-n times, but ChildB only 0-1 times. (Actually I need several nodes of each type, but if I can do it for ChildA and ChildB, extending it to ChildX etc. and ChildY etc. should be simple). There should be no order constraint. I'd appreciate any help. Any links that explain schema indicators in depth would also be helpful.</p>