@Override
    protected void appendBodyString(StringBuilder buffer) {
        if (items().isEmpty()) return;
        if (items().size() == 1) {
            buffer.append(items().get(0));
            return;
        }
        for (int i = 0; i < items().size(); i++) {
            if (i == 0 && items().get(i) instanceof TrueItem) continue; // skip positive true

            buffer.append(i == 0 ? "+" : "-").append(items().get(i));
            if ( i < items().size() - 1)
                buffer.append(" ");
        }
    }
Overridden to skip the positive TrueItem and (otherwise) append "+"
to the first item and "-" to the rest
container-search/src/main/java/com/yahoo/prelude/query/NotItem.java