- problem generated by expat version
/etc/apt/sources.list.d/freecad-temp.list
containing
deb http://security.ubuntu.com/ubuntu xenial-security main universe
- then
apt-get update
apt install libexpat1=2.1.0-7ubuntu0.16.04.3
apt install expat=2.1.0-7ubuntu0.16.04.3
apt install libexpat1-dev=2.1.0-7ubuntu0.16.04.3