It seems that much research concerning corrective feedback in SLA is assuming a recasts-prompts dichotomy, and researchers are willing to further examine recasts in terms of implicitness/explicitness. But research concerning sub-categories of prompts (repetition, elicitation, clarification request, metalinguistic clue) is much less (or maybe it's because I haven't read enough; if that's the case, will you please provide some research concerning these four finer types of corrective feedback?). Is my impression correct? If so, what may be the possible reasons for such research preference?