Right way of specifying engraver name?

[Francisco Vila]

Hello,

in the following score for 2.16

\score {
  \new PianoStaff <<
    \new Staff
    \crossStaff { <c' e'> q }
    \new Staff
    \crossStaff { \clef bass <c a,> q }
  >>
}

\layout {
  \context {
    \PianoStaff
    \consists #Span_stem_engraver
  }
  \context {
    \Staff
    \remove "Bar_engraver"
  }
}

%end

I have to name engravers by a different convention. IIRC all engravers
should be double quoted, but Span_stem_engraver does not seem to admit
this. What am I doing wrong?

-- 
Francisco Vila. Badajoz (Spain)
www.paconet.org , www.csmbadajoz.com