\version "2.18.2"

right = << \relative c' {
  \tempo "Gracefully"
  \clef "treble"
  \key c \major
  \numericTimeSignature \time 3/4
  e4( g2)  |%m1/right
  g4( e2)  |%m2/right
  f4( d c  |%m3/right
  \change Staff = left
  b2.)     |%m4/left
  %\change Staff = right
  %R2.      |%m4/right
}
  \addlyrics {
    Rid -- ing | riding, | 'round and a round
  }
>>

left = \relative c' {
  \clef "bass"
  \key c \major
  \numericTimeSignature \time 3/4
     R2. |%m1/left
     R2. |%m2/left
     R2. |%m3/left
     %b2. |%m4/left -> changed to left from right  
}


pianoPart = \new PianoStaff \with { 
  instrumentName = "Pno."
} <<
 \new Staff = "right" \right
 \new Staff = "left" \left
>>


\score {
 <<
 \pianoPart
 % ...
 >>
}