&KEY markers in DEFSETF lambda lists are supported, but the corresponding keywords must appear literally in the program text.

An attempt to modify read-only data SIGNALs an ERROR. Program text and quoted constants loaded from files are considered read-only data. This check is only performed for strings, not for conses, other kinds of arrays, and user-defined data types.

See also Section 31.11.2, “Macros EXT:LETF & EXT:LETF*”.