transform(value) {
    if (Array.isArray(value)) {
      value.forEach(this.transform, this);
    } else {
      this.tag.transformers.push(normalizeHook(this.dirname, value));
    }
  }
If a custom tag has an associated transformer then the transformer
will be called on the compile-time Node. The transformer can manipulate
the AST using the DOM-like API to change how the code gets generated.