Photo of a <a href="https://en.wikipedia.org/wiki/Source_four" class="extiw" title="w:Source four">Source four</a> by <a href="//commons.wikimedia.org/wiki/User:KeepOnTruckin" title="User:KeepOnTruckin">User:KeepOnTruckin</a>
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CC BY 2.5
No machine-readable source provided. Own work assumed (based on copyright claims).