In our problem here, when a lecturer discusses a bullet, the words used are likely to be different from those used in another bullet, suggesting that the spoken documents themselves, when ignoring the alignment model above for the time being, could potentially indicate the semantic boundaries that we are interested in here.
the cohesion conveyed by the repetition of the words that appear in transcripts but not in slides could be additionally helpful; this is very likely to happen considering the significant imbalance of text lengths between bullets and transcripts, from which the alignment models by themselves may suffer.