The Nil object is a special object that does not contain data. It usually represents the absence of an object, as a null string represents a string with no characters. It has only the methods of the Object class. Note that you use the Nil object (rather than the null string ("")) to test for the absence of data in an array or other Collection class entry:

if .nil = board[row,col]   /* .NIL rather than "" */
then ...