舉證責任 證明度 證據提出 說服責任 證明妨礙 表見證明 限縮爭點 舉證責任分配 不當得利 票據原因關係抗辯 Burden of proof Stand of proof Production of evidence Burden of persuasion Spoliation of evidence Prima facie evidence Issue-narrowing Allocation of the burden of proof Unjust enrichment Defense to negotiable instruments claims