private static String extractBooleanExpression(String line) {
        String field = "\"boolean\":";
        var start = line.indexOf(field);
        var end = line.indexOf("\"", start + field.length() + 1);
        return line.substring(start + field.length() +1 , end);
    }
Parses a feed file containing documents in JSON format. Its implementation is based on the following assumptions:
 1. Each document has single predicate field.
 2. The predicate is stored in a field named "boolean".
 3. There is just one "boolean" field on each line

 @author bjorncs
predicate-search/src/main/java/com/yahoo/search/predicate/utils/VespaFeedParser.java