By way of introduction to this topic, the following sets out some of the principles that have emerged from the case law reported in the sub-sections below. The EPC requires clear requests with regard to testimonies which a party wishes witnesses to give. A party who wishes to adduce witness evidence should indicate what factual details it wishes to prove by this means. The function of a witness is to corroborate what has been alleged and not to fill in the gaps in facts. Additional clarifications provided by a witness to close a potential gap in the documentary evidence on file cannot be considered per se new facts; hearing a witness would otherwise be futile. The parties must be given an opportunity to comment on the results of the witness hearing. The EPC does not preclude the parties to the proceedings from offering their employees as witnesses. An allegation that a witness might be biased does not in itself render their testimony inadmissible; rather, suspicion of bias is a matter to be considered when evaluating the evidence. All the means of giving or taking evidence covered by Art. 117 EPC are subject to the discretion of the department concerned but if the evidence offered is decisive for proving contested facts on which the case turns, the department must, as a rule, order that it be taken.