«لم يقم زيدٌ ثم قام» is coherent: he did not stand at an earlier point but stood later. «لمّا يقم زيدٌ ثم قام» clashes in a single report when «lamma» is asserting non-occurrence up to the present moment.
The important feature is the continuation of non-occurrence up to the reference point, not merely the negation of one isolated interval in the past.
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Al-Itlaq wa al-Taqyid: Rhetorical Functions of Unrestricted and Qualified Expression
«لم يقم زيدٌ ثم قام» is coherent: he did not stand at an earlier point but stood later. «لمّا يقم زيدٌ ثم قام» clashes in a single report when «lamma» is asserting non-occurrence up to the present moment.
The important feature is the continuation of non-occurrence up to the reference point, not merely the negation of one isolated interval in the past.