nsgmls:0:1:59:W: cannot generate system identifier for public text "-//OASIS//DTD DocBook V4.1//EN" nsgmls:0:6:0:E: reference to entity "REFENTRY" for which no system identifier could be generated nsgmls:0:1:0: entity was defined here nsgmls:0:6:0:E: DTD did not contain element declaration for document type name po4a::sgml: Error while running nsgmls -p. Please check if nsgmls and the DTD are installed.