Consider the premise. Does the hypothesis naturally and logically follow? Premise: That is to say, beside Indo-European and Indo-Hittite, there must have existed an Indo-Slurvian. Hypothesis: There must have existed an Indo-Slurvian.
That is to say, beside Indo-European and Indo-Hittite, there must have existed an Indo-Slurvian.###There must have existed an Indo-Slurvian.