public Map<String, String> getQueryMap() {
        Map<String, String> ret = new HashMap<>();
        for (NowQueryExpression expression : expressionMap.values()) {
            ret.put(expression.getDocumentType(), expression.toString());
        }
        return ret;
    }
Class which converts a selection tree into a set of queries per document type.
If unsupported operations are or illegal arguments are encountered, an exception is thrown.

@author Ulf Lilleengen
document/src/main/java/com/yahoo/document/select/convert/SelectionExpressionConverter.java