A comprehensive list of Prolog instantiation modes is given in a previous question: Meaning of instantiation mode indicators in arguments of Prolog predicates.
However, I have not been able to find any reference to or explanation about the double plus (++) instantiation mode in ECLiPSe Prolog. An example predicate in the ECLiPSe docs that uses the '++' mode is setval(++,?).
Does anyone know what the meaning is of this mode?
This mode is described in http://eclipseclp.org/doc/bips/kernel/compiler/mode-1.html as having the following meaning: ++ The argument is ground.