knuth-let­ter – Knuth's ex­am­ple let­ter macros

Th­ese are Knuth’s own let­ter macros. Doc­u­men­ta­tion is en­tirely miss­ing from the archive, but the macros are dis­cussed in some de­tail in the TeXbook (which says it is de­scrib­ing let­ter­for­mat.tex).