Suppressing LilyPond clashing NoteColumn warnings

Stacked LilyPond voices can produce clashing NoteColumn warnings.

   \new Staff <<
      \new Voice {
         \override Stem #'direction = #up
         c''4 c''4 c''4 c''4
      }
      \new Voice {
         \override Stem #'transparent = ##t
         \override Stem #'length = #0
         \override NoteHead #'duration-log = #1
         e'4 e'4 e'4 e'4
      }
      \new Voice {
         \override Stem #'direction = #down
         c'4 c'4 c'4 c'4
      }
   >>


   Preprocessing graphical objects...
   348.ly:6:27: warning: ignoring too many clashing note columns
           } { c''4 c''4 c''4
                              c''4 }
   348.ly:6:22: warning: ignoring too many clashing note columns
           } { c''4 c''4
                         c''4 c''4 }
   348.ly:6:17: warning: ignoring too many clashing note columns
           } { c''4
                    c''4 c''4 c''4 }
   348.ly:6:12: warning: ignoring too many clashing note columns
           } {
               c''4 c''4 c''4 c''4 }

What's going on here is that the middle note of each 'chord' sets Stem #'transparent = ##t which renders stems transparent but causes clashing note column warnings.

The solution is to set \override NoteColumn #'ignore-collision = ##t somewhere in the score.