public Stream<ConceptMap> materialise(ConceptMap answer) {
        return this.withSubstitution(answer)
                .getAtom()
                .materialise()
                .map(ans -> ans.explain(answer.explanation()));
    }
public Stream < ConceptMap > materialise ( ConceptMap answer ) { return this . withSubstitution ( answer ) . getAtom ( ) . materialise ( ) . map ( ans -> ans . explain ( answer . explanation ( ) ) ) ; }
materialise  this query with the accompanying answer - persist to kb
@param answer to be materialised
@return stream of materialised answers