/usr/share/cairo-dock/separator.desktop is in cairo-dock-data 3.3.99.beta1.2.really.3.3.2-0ubuntu2.
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#[gtk-about]
[Desktop Entry]
#F[Icon]
frame_maininfo=
#d+ Name of the container it belongs to:
Container =
#v
sep_display=
#> Separators' appearance is defined in the global configuration.
Icon=
#X[Extra parameters]
frame_extra =
#f[0;100] Order you want for this launcher among the others:
Order=0
Icon Type = 2
Type = Separator
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