MPICommandExpr

 typedef struct MPICommandExpr {
    MPICommandOperator	oper;
    MPICommandAddress	address;
	union {
	    MPICommandConstant	value;	/* [*'address'] 'oper' ['value'] */
	    MPICommandAddress	ref;	/* [*'address'] 'oper' [*'ref']  */
	} by;
} MPICommandExpr;

CommandExpr is a structure that represents an expression for an MPICommand object.