afterGrace on Completion_heads_engraver layout

[Venus' Wink]

Hi,

I'm trying use \afterGrace on layout consists Completion_heads_engraver as:

\version "2.18.2"
\layout {
    \context {
        \Voice
        \remove "Note_heads_engraver"
        \consists "Completion_heads_engraver"
    }
}
{
    c'4 \afterGrace a4 b16
}

which issues attached test.pdf.
The a4 also appears in the grace.

The image I expect is the other attachment expected.pdf yielded 
with \context brace includes only \Voice.

How can an image looks same be got without modifying the \context
brace above?

Thanks
Venus' Wink

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test.pdf
Description: Adobe PDF document

expected.pdf
Description: Adobe PDF document