Truncate beginning of a file

[s0suk3]

file.truncate(X) will truncate the file to at most X bytes (i.e. leave
the first X bytes of the file and throw away the rest). Is there a way
to throw away, say, the first X bytes of the file, and leave the rest?
(Without opening the same file for reading, reading and processing,
overwriting the file with the new processed data, etc.)
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