The Unicode Byte Order Mark (BOM) is treated as whitespace in the default readtable. This allows the Lisp reader to read a Unicode (UCS-2 encoded,
external-format
:unicode
) file regardless of whether the BOM is present.
Some editors including Microsoft Notepad and the LispWorks editor write the BOM when writing a file with Unicode (UCS-2) encoding.
LispWorks User Guide and Reference Manual - 21 Dec 2011