imagemagick-6.q16 built without DJVU

[Jeffrey Ratcliffe]

imagemagick-6.q16:
  Installed: 8:6.9.7.4+dfsg-16ubuntu6.4
  Candidate: 8:6.9.7.4+dfsg-16ubuntu6.4
  Version table:
 *** 8:6.9.7.4+dfsg-16ubuntu6.4 500
        500 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages
        100 /var/lib/dpkg/status
     8:6.9.7.4+dfsg-16ubuntu6 500
        500 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages

convert creates PPM instead of DjVu:

$ convert rose: test.djvu
$ identify test.djvu
test.djvu PPM 70x46 70x46+0+0 8-bit sRGB 9.67KB 0.000u 0:00.000

if I feed identify with a real djvu, it cannot recognise it:

$ convert rose: test.jpg
$ c44 test.jpg test.djvu
$ identify test.djvu
identify-im6.q16: no decode delegate for this image format `DJVU' @ error/constitute.c/ReadImage/504.

imagemagick, properly configured, can read and write djvu images and I would expect the Ubuntu package to do so.