Why are the ontologies initially produced in OBO flat file format instead of XML?

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The ontologies are initially produced in the specially designed OBO flat file format. They are converted to XML once a month for the convenience of users who require this facility. Both formats and many others are available in the GO downloads section. We use the OBO flat file format because it is human-readable, and also because the file is much smaller without the XML tags. This means that it is much quicker and easier for the curators to handle the file on a day-to-basis.