\version "2.17.26"

\include "LilyJAZZ.ily"

stemOff = { \override Staff.Stem.stencil = ##f }
stemOn  = { \revert Staff.Stem.stencil }

improvMusic = \relative b' {
    \improvisationOn \stemOff
    b4 b b b  | b b b b
    b4 b b b  | b b b b
    \stemOn \improvisationOff
} % improvMusic

\score {
  \new Staff
  <<
    \improvMusic
    { \jazzOn s4 \jazzOff }
  >>
}