src/language-yaml/print-preprocess.js/defineShortcuts
function defineShortcuts(node) {
  switch (node.type) {
    case "document":
      defineShortcut(node, "head", () => node.children[0]);
      defineShortcut(node, "body", () => node.children[1]);
      break;
    case "documentBody":
    case "sequenceItem":
    case "flowSequenceItem":
    case "mappingKey":
    case "mappingValue":
      defineShortcut(node, "content", () => node.children[0]);
      break;
    case "mappingItem":
    case "flowMappingItem":
      defineShortcut(node, "key", () => node.children[0]);
      defineShortcut(node, "value", () => node.children[1]);
      break;
  }
  return node;
}