package.mask file can be used to hard mask packages or
certain versions of packages that should not be merged by users.
This is often used when adding certain experimental (either in ebuild
or upstream terms) packages to the tree, or to prevent merging of
packages that are broken or break something else. Every entry should
have a comment detailing the specific reason for the mask. The format
package.mask file is described in
This file can be used in subprofiles to mask packages only for certain setups.