XkbSASetGroup sets the group_XXX field of act from the group index grp.
Actions associated with the XkbGroupAction structure change the current group state when keys are pressed and released:
typedef struct _XkbGroupAction {
unsigned char type; /* XkbSA_{Set|Latch|Lock}Group */
unsigned char flags; /* with type, controls the effect on groups */
char group_XXX; /* represents a group index or delta */
} XkbGroupAction;
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