Validation of field inside a repeater.

Hi, I would like to do the following scenario where I've a repeater that have a repetition of a block of "A/B/C OR D". If D is set, A/B/C shouldn't trigger any validation, if D is not set, A/B/C should trigger a validation (required). if d is not set, and a/b/c not set, should trigger a 'required' validation. Any idea on how to achieve this ?