Issues with getting search results

[szymach]

Hello,
I was researching this library for use in one of my projects and wanted to try out the search engine. Following the README I used the Docker images to index the files located in the test directory of this repository (similairly to how it is done in scripts/github-test). Then I extracted a part of a formula from test/exact-match.xhtml:

<math xmlns="http://www.w3.org/1998/Math/MathML">
    <semantics>
        <mrow>
            <msup>
                <mi>x</mi>
                <mo>′</mo>
            </msup>
            <mo>,</mo>
            <msup>
                <mi>y</mi>
                <mo>′</mo>
            </msup>
            <mo>,</mo>
            <msup>
                <mi>z</mi>
                <mo>′</mo>
            </msup>
            <mo>,</mo>
            <msup>
                <mi>t</mi>
                <mo>′</mo>
            </msup>
        </mrow>
    </semantics>
</math>

but it yields no results. Am I misunderstanding something? What kind of a dataset does the indexing engine actually expect? Are the .xhtml files OK to use or do I need a different file format? Do I have to supply a specific index name when indexing data? Thank you for any help in advance.