Algebreic Programming in Opal. - Andreas Fett; Carola Gerke; Wolfgang Grieskamp; Peter Pepper (Bull. EATCS, 1993)

OPAL: Design and Implementation of an Algebraic Programming Language. - Klaus Didrich; Andreas Fett; Carola Gerke; Wolfgang Grieskamp; Peter Pepper (Programming Languages and System Architectures, 1994)