Halliday & Matthiessen (2004: 339, 340):
Since the tense system is recursive, there should be no longest possible tense. However, in practice there are certain restrictions which limit the total set of those that occur. These restrictions, or ‘stop rules’, are as follows:
- (i) Apart from alpha, future occurs only once.
- (ii) Apart from alpha, present occurs only once, and always at the deepest level.
- (iii) Apart from alpha, the same tense does not occur twice consecutively. …These restrictions limit the total number of finite tenses to 36.