Cleartool will prompt for a comment if you do not use the third format to supply one.
Although you may not be used to providing a description of the changes at checkout time ot is good practice to enter the reason you are checking out the file.
The package allows user to perform bulk operations of getting data from repository and committing them back.
It's simple enough to have a stream in Clear Case coexist with a Mercurial repository.
Files in Clear Case can only be changed when they are checked out.
For almost any function, that works with current file (they have suffix ), that either allow modification of file, either switch it to read-only state.
So hijacking a file is a description of the process where you change the read bit on a file from read-only to modify on a file that is actually in source control.
This technique is only available in snapshot views - files in dynamic views can not be hijacked.
All you need is to create a snapshot view on the stream in question.
After the files are updated and on a local disk, turn the snapshot view into a Mercurial repository.