public boolean isSetComparison() {
    if ("=~".equals(operation) && children.size() == 2) {
      Object value1 = children.get(0).getValue();
      Object value2 = children.get(1).getValue();

      return value1 instanceof Set && value2 instanceof Set;
    }
    return false;
  }
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