private void collectPostfix(List<ExpressionInfo> collector, boolean skipIrrelevant) {
    for (ExpressionInfo expr : children) expr.collectPostfix(collector, skipIrrelevant);
    if (!skipIrrelevant || isRelevant()) collector.add(this);
  }
Examples where operation is null:
- GString method name: foo."$bar"()
- Argument list (has children but no operation)
spock-core/src/main/java/org/spockframework/runtime/model/ExpressionInfo.java