static NLABlock nla_simplifyIntroductions(NLABlock nla) {
  nla_parse(nla);
  for (NLABlock.Line line : cloneList(nla.introductions())) {
    // simplify introduction
    line.proposition = frag("var *", line.introducedEntity);
    
    // introduce a new proposition
    NLABlock.Line prop = nla.new Line;
    prop.proposition = frag(line.introducedEntity, "* is *", line.introducedEntity.firstSynonym());
    nla.parsedLines.add(line.index()+1, prop);
  }
}