diff --git a/examples/book/book.capnp b/examples/book/book.capnp
index c9c13ac..1e759de 100644
--- a/examples/book/book.capnp
+++ b/examples/book/book.capnp
@@ -20,6 +20,13 @@ struct Nulldata $C.mapname("nulldata_t") {
   null  @0: UInt32 $C.mapname("null_");
 }
 
+struct Buy $C.mapname("buy_t") {
+  from  @0: Text;
+  u :union $C.mapname("u") $C.mapuniontag("with_recipe") {
+    norecipe   @1: Void;
+    recipeAddr @2: Text $C.mapname("recipe_addr");
+  }
+}
 struct Book $C.mapname("book_t") {
   title   @0: Text;
   authors @1: List(Text) $C.mapname("authors") $C.maplistcount("n_authors");
@@ -29,7 +36,7 @@ struct Book $C.mapname("book_t") {
   magic1  @2: List(UInt32) $C.mapname("magic_1") $C.maplistcount("n_magic1");
   description @8: Text;
   acquire :union $C.mapuniontag("acquire_method") {
-    buy   @3: Text;
+    buy   @3: Buy;
     donation @4: Text;
   }
 }
diff --git a/examples/book/book.h b/examples/book/book.h
index cee7016..91a9b9f 100644
--- a/examples/book/book.h
+++ b/examples/book/book.h
@@ -6,36 +6,44 @@
 #include <string.h>
 
 typedef struct {
-    char *caption;
-    int32_t start;
-    int32_t end;
+  char *caption;
+  int32_t start;
+  int32_t end;
 } chapter_t;
 
 typedef struct {
-    uint64_t isbn;
-    uint32_t year;
+  uint64_t isbn;
+  uint32_t year;
 } publish_t;
-    
+
 typedef struct {
-    int null_;
+  int null_;
 } nulldata_t;
 
 typedef struct {
-    char *title;
-    int   n_authors;
-    char **authors;
-    int   n_chapters;
-    chapter_t **chapters_;
-    publish_t *publish;
-    nulldata_t *nulldata;
-    int   n_magic1;
-    uint32_t *magic_1;
-    char *description;
-    int   acquire_method;
-    union {
-	char *buy;
-	char *donation;
-    } acquire;
+  char *from;
+  int with_recipe;
+  union {
+    char *recipe_addr;
+  } u;
+} buy_t;
+
+typedef struct {
+  char *title;
+  int n_authors;
+  char **authors;
+  int n_chapters;
+  chapter_t **chapters_;
+  publish_t *publish;
+  nulldata_t *nulldata;
+  int n_magic1;
+  uint32_t *magic_1;
+  char *description;
+  int acquire_method;
+  union {
+    buy_t *buy;
+    char *donation;
+  } acquire;
 } book_t;
 
 #endif
diff --git a/examples/book/test.c b/examples/book/test.c
index 6e1c560..7e6272f 100644
--- a/examples/book/test.c
+++ b/examples/book/test.c
@@ -38,6 +38,13 @@ int encode() {
 	.isbn = 335677,
 	.year =2001
     };
+    buy_t buy = {
+	.from = "Xinghua Book store",
+	.with_recipe = Buy_u_recipeAddr,
+	.u = {
+	    .recipe_addr = "Xinghua Street"
+	}
+    };
     struct capn_segment *cs;
     struct Book b;
     Book_ptr p;
@@ -53,7 +60,7 @@ int encode() {
     book.magic_1 = &magic1[0];
     book.description = NULL;
     book.acquire_method = Book_acquire_buy;
-    book.acquire.buy = "bought from Xinhua book store";
+    book.acquire.buy = &buy;
 
     capn_init_malloc(&c);
     cs = capn_root(&c).seg;
@@ -106,7 +113,11 @@ int decode() {
     }
 
     if (book->acquire_method == Book_acquire_buy) {
-	printf("%s\n", book->acquire.buy);
+	printf("buy from %s\n", book->acquire.buy->from);
+	if (book->acquire.buy->with_recipe == Buy_u_recipeAddr) {
+	    printf("recipe address %s\n",
+		   book->acquire.buy->u.recipe_addr);
+	}
     }
     else {
 	printf("%s\n", book->acquire.donation);