LilyPond stem direction

LilyPond Stem #'direction takes #up, #down or a procedure.

   \new Staff {
      c'4
      \once \override Stem #'direction = #ly:stem::calc-direction
      c'4
      c'4
      c'4
   }

   \layout {
      \context { \Staff
          \override Stem #'direction = #down } }

Here Stem #'default-direction is #down and so the procedure #ly:stem::calc-direction is helpful.