PR# 12836 Precondition failure for `is_reasable' on CONSOLE

In one of my example projects, I build a version of 'cat'. For that project, I have to read from an io medium, either a file or stdin.  While explaining that stdin is a CONSOLE and CONSOLE is a PLAIN_TEXT_FILE, it discovered that my precondition of accessibility (is_readable) causes a downstream exception because  there is no such file or directory (in the eventual call to file_stat).

I think is_readable should be redefined in CONSOLE but the question is how? True would not be good since sometime you cannot read.