 Even acts of a witness which obstructed proceeding during cross-examination, may be held to be material.In Millward, where a police officer denied having sought the assistance of his colleague in identifying in the court room a person charged with driving offences, that act was held by the English Court of Appeal as material statement in that it brought to a halt a line of cross-examination, went to the heart of the case, in that the stopping of the cross-examination might very have affected the outcome of the case.However, questions asked during cross-examination, which purpose is to impugn the credibility of a witness as to his previous convictions have been held not to be material.