current_context_descending.ly

% There is an exception to this general rule: inside of an {…} construct (sequential music), the construct’s notion of the “current context” will descend whenever an element of the sequence ends in a subcontext of the previous current context. This avoids spurious creation of implicit contexts in a number of situations but means that the first context descended into will be kept alive until the end of the expression.
% In contrast, the contexts of a <<…>> construct’s (simultaneous music) expression are not carried forth, so enclosing a context creating command in an extra pair of <<…>> will keep the context from persisting through all of the enclosing {…} sequence.
% - http://lilypond.org/doc/v2.19/Documentation/notation/keeping-contexts-alive.html

\version "2.19.81"

\score {
  \new Staff {
    \new Voice {
      \override NoteHead.stencil = ##f
      c'4
    }
    d'4 % This NoteHead is also omitted
  }
}

\score {
  \new Staff {
    \new Voice {
      \override NoteHead.stencil = ##f
      c'4
    }
    \new Voice {
      d'4 % This NoteHead is present
    }
  }
}

\score {
  \new Staff {
    <<\new Voice {
      \override NoteHead.stencil = ##f
      c'4
    }>>
    d'4 % This NoteHead is present
  }
}