The NIL object is a special environment object that does not contain data. It represents the absence of an object, the way a null string represents a string with no characters. Its only methods are those of the Object class. You use the NIL object (rather than the null string) to test for the absence of data in an array entry:
if board[row,column] = .nil then ...
All the environment objects REXX provides are single symbols, but use compound symbols when you create your own, to avoid conflicts with future REXX-defined entries.
Objects in the Environment object--also called the Global environment object--are available to all programs running on your system. Objects in the Local environment object (below) are available to programs running within the same process.