2. The method of claim 1 , further comprising: (d) interpreting first speech signals as the interpreted text string, wherein (d) is performed prior to (a); and in (b), conditional upon a determination that the respective sub-tree is inaccurate, obtaining replacement speech signals for replacing the respective sub-tree and the respective sub-tree's corresponding section.
3. The method of claim 2 , further comprising: performing (d) to (b) for the replacement speech signals.