jembie

In [lecture.tex-18](https://github.com/knowsys/TheoLog/blob/master/Vorlesungen/lecture-18.tex), namely line 244 (see: [here](https://github.com/knowsys/TheoLog/blob/master/Vorlesungen/lecture-18.tex#L244)) is a minor typo, instead of 'ersetzten' it should be 'ersetzen', as also mentioned in the lecture while explaining the procedure (see: [Substitution...