/* types, of pointers to corresponding structs */
typedef struct A_stm_ *A_stm;
typedef struct A_exp_ *A_exp;
typedef struct A_expList_ *A_expList;

/* binary operation */
typedef enum { A_plus, A_minus, A_times, A_div } A_binop;

/* statement */
struct A_stm_ {
	/* types: compound, assign, print */
	enum { A_compoundStm, A_assignStm, A_printStm } type;
	union {
		struct { A_stm stm1, stm2; }	compound;
		struct { char *id; A_exp exp; }	assign;
		struct { A_expList exps; }		print;
	} u;
};

/* statement constrctor functions */
A_stm A_CompoundStm	( A_stm stm1, A_stm stm2 );
A_stm A_AssignStm	( char *id, A_exp exp );
A_stm A_PrintStm	( A_expList exps );

/* expression */
struct A_exp_ {
	/* types: id, number, operation, sequence */
	enum { A_idExp, A_numExp, A_opExp, A_eseqExp } type;
	union {
			char *id;
			int num;
			struct { A_exp exp1; A_binop op; A_exp exp2; } op;
			struct { A_stm stm; A_exp exp; } eseq;
	} u;
};

/* expression constructor functions */
A_exp A_IdExp	( char *id );
A_exp A_NumExp	( int num );
A_exp A_OpExp	( A_exp exp1, A_binop op, A_exp exp2 );
A_exp A_EseqExp	( A_stm stm, A_exp exp );

/* expression list */
struct A_expList_ {
	/* types: pair, last */
	enum { A_pairExpList, A_lastExpList } type;
	union {
		struct { A_exp head; A_expList tail; } pair;
		A_exp last;
	} u;
};

/* expression list constructor functions */
A_expList A_PairExpList (A_exp head, A_expList tail);
A_expList A_LastExplist (A_exp last);