classic confinement requires the core snap to be installed. Install it by running `snap install core`.
~/.workspace/ipe-tools/snap[master]$ sudo snap install core
error: cannot perform the following tasks:
- Mount snap "core" (888) (cannot install core snap "core" when core snap "ubuntu-core" is already present)
~/.workspace/ipe-tools/snap[master]$ sudo snap remove ubuntu-core 
error: cannot remove "ubuntu-core": snap "ubuntu-core" is not removable
~/.workspace/ipe-tools/snap[master]$ snap list
Name         Version  Rev   Developer  Notes
ipe          7.2.7    x1               -
ubuntu-core  16.04.1  1411  canonical  -