M DOs, XECUTEs, and extrinsics add a level to the invocation stack. Matching QUITs take a level off the stack. When GT.M executes either of these commands, an extrinsic function, or an extrinsic special variable, it "pushes" information about the new environment on the stack. When GT.M executes the QUIT, it "pops" the information about the discarded environment off the stack. It then reinstates the invoking routine information using the entries that have now arrived at the active end of the stack.
![[Note]](images/note.png)
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In the M stack model, a FOR command does not add a stack frame, and a QUIT that terminates a FOR loop does not remove a stack frame. |