WHEN takes an implicit PROGN like that (so does UNLESS). IF can't because it needs to delimit the then-form from the else-form somehow. http://clhs.lisp.se/Bod

WHEN takes an implicit PROGN like that (so does UNLESS). IF can't because it needs to delimit the then-form from the else-form somehow. http://clhs.lisp.se/Body/m_when_.htm http://clhs.lisp.se/Body/m_when_.htm