The SI (System Intrinsics) command changes the file used for the system intrinsics or displays the current title of that file.
When the system intrinsics file is changed, currently active tasks continue to use the old intrinsics stack; new jobs use the new intrinsics stack.
Syntax

Explanation
SI
Displays the title of the file containing the current system intrinsics.
SI +
Causes the default file, SYSTEM/INTRINSICS, to be used for the system intrinsics.
SI <file title>
Causes <file title> to become the file used for the system intrinsics.
Examples
Example 1
SI SYSTEM/X
SYSTEM/X WILL BE LOADED
MSG
--Mix--Time--------------- MESSAGES ------------------- 6029 22:11 SYSTEM/X HAS BEEN LOADED
Example 2
SI +
SYSTEM/INTRINSICS WILL BE LOADED
MSG
--Mix--Time--------------- MESSAGES ------------------ 6039 22:13 SYSTEM/INTRINSICS HAS BEEN LOADED
Example 3
SI
INTRINSICS: SYSTEM/INTRINSICS