How to Implement Recursive data type

I want to Implement abstract syntax tree in mojo, so far no luck,
Something simple as this in C :
https://github.com/orangeduck/mpc/blob/master/mpc.h#L291

struct ast {
  char *tag;
  char *contents;
  int children_num;
  struct ast** children;
}

struct ast {
  char *tag;
  char *contents;
  int children_num;
  struct ast** children;
}

struct ast {
  char *tag;
  char *contents;
  int children_num;
  struct ast** children;
}

struct ast {
  char *tag;
  char *contents;
  int children_num;
  struct ast** children;
}

GitHub

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