question: Given the following triplets constituting a sub-graph, please infer the relationship between "sentence generation" and "natural language generation".
  Triplets: story ending generation - Hyponym-Of - natural language generation; story ending generation - Is-a-Prerequisite-of - sentiment control; natural language generation - Conjunction - natural language understanding; Part-of; Conjunction; Evaluate-for; Is-a-Prerequisite-of; Used-for; Hyponym-Of
  Relationships Types: Compare, Part-of, Conjunction, Evaluate-for, Is-a-Prerequisite-of, Used-for, Hyponym-Of