%.S

Normally, commands/data are deleted from the stack as soon as they have been read from the stack.

This command causes the next INPUT statement to read the topmost entry from the stack without deleting it. The entry is treated as input data, no matter whether it actually is data or a command.

This allows you check a stack entry and then, depending on its content, decide whether to process it or not.

To check the contents of the stack, you can also use the system variable *DATA.

For further information on the Natural stack, see the section Stack Processing (in the Programming Guide).