type expr = union
	`Add (expr#, expr#)
	`Mul (expr#, expr#)
	`Val int
;;

cosnt fold = {expr
	match expr
	| `Plus (`Val 0, x): -> fold(x)
	| `Plus (x, `Val 0): -> fold(x)
	| `Mul (`Val 0, `Val _): -> `Val 0
	| `Mul (`Val _, `Val 0): -> `Val 0
	| other:	-> other
	;;
}

Roughly equivalent to....


enum {
	Add,
	Mul,
};

struct Expr {
	int op;
	union {
		struct {
			Expr *left;
			Expr *right;
		};
		int val;
	};
};

Expr fold(Expr *e){
	switch(op){
	case Plus:
		switch(e->left->op){
		case Val:
			if(e->left->val == 0)
				return e->right;
			break;
		default:
			switch(e->right->op){
			case Val:
				if(e->right->val == 0)
					return e->left;
				break;
			default:
				return e;
			}
	...