public ConfigDiff guess() {
            if (inputPath == null) {
                throw new IllegalStateException("inputPath must be set");
            }

            // in: config
            ConfigSource inConfig = newConfig()
                    .set("type", "file")
                    .set("path_prefix", inputPath.toAbsolutePath().toString());
            inConfig.set("parser", parserConfig);

            // config = {exec: execConfig, in: inConfig}
            ConfigSource config = newConfig()
                    .set("exec", execConfig)
                    .set("in", inConfig);

            // embed.guess calls GuessExecutor and returns ConfigDiff
            return embed.guess(config).getNested("in").getNested("parser");
        }
This method returns PreviewResult.

@return PreviewResult returns the result by PreviewExecutor
@throws IOException in case of failures in file operations
embulk-junit4/src/main/java/org/embulk/test/TestingEmbulk.java