信用证条款：FM：HANG SENG BANK，HONGKONGTO：BANK OF CHINA，NANJINGDD：180206L/C NO．1234L/C AMT：USD10，000．00APPLICANT：XYZ COMPANY，HONGKONGBENEFICIARY：ABC COMPANY，NANJINGDESCRIPTION OF GOODS AND/OR SERVICES：MEN’S SLACKSPRICE TERMS：CIF HONGKONGDOCUMETNS REQUIRED：COMMERCIAL INVOICE IN 6 COPIES CERTIFYING THE COUNTRY OF ORIGIN IS CHINA．信用证未对发票做任何其他规定。受益人的有权签字人为陈丹。请问发票应怎样签署？