程序代码：

typedef struct a_stm_ *a_stm;
typedef struct a_explist *a_explist;
struct a_stm_ 
{
emu{a,b,c}kind;
union
{
  struct{a_stm stm1,stm2}compound;  

 struct{a_explist exps}print;
}u;
};
a_stm a_compoundstm(a_stm stm1,a_stm stm2);

程序代码：

a_stm a_compoundstm(a_stm stm1,a_stm stm2)
{
    a_stm s=checked_malloc(sizeof(*s));
    s->kind=a_compoundstm;
    s-> s-> return s;
}
创建一个构造器，用malloc为此数据结构分配空间并初始化