#!/usr/local/bin/csi -script

(use posix)

(let ((p (open-input-file "/etc/hosts")))
  (read-char p)
  (read-char p)
  (read-char p)
  (set-file-position! p -1 seek/cur))

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Error: (set-file-position!) invalid negative port position
-1
#<input port "/etc/hosts">