Idmaj folds a second purpose into the wording already serving the first; a distinct thematic departure is not necessary. Istīṭrād actually departs from the main line to a related topic and later returns.
For example, introducing a historical incident while praising generosity and then resuming that praise is a digression-and-return pattern. Describing generosity in a way that also implies eloquence inside the same expression is closer to idmaj.
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Ilm al-Badiʿ: The First Nine Semantic Rhetorical Devices
Idmaj folds a second purpose into the wording already serving the first; a distinct thematic departure is not necessary. Istīṭrād actually departs from the main line to a related topic and later returns.
For example, introducing a historical incident while praising generosity and then resuming that praise is a digression-and-return pattern. Describing generosity in a way that also implies eloquence inside the same expression is closer to idmaj.