int MailBoxes::boxIndex(int box)
//postcondition: returns the index of the element in myBoxes
//               whose POBox field equals box if there is such an element;
//               otherwise, returns -1.
{
    int k;
    for(k=0; k < myBoxCount; k++)
    {
	if (myBoxes[k].POBox == box)
	{
	    return k;
	}
    }
    return -1;
}