When is a command-line tool that watches a file or directory for modification
and executes a command in response to that.
It currently runs on Linux over x86-64.
When requires a nop [1] binary to found in $PATH and that nop can find
clib. This can be achieved by
% export NOPLIBS=/path/to/nop/libs
To execute a command in response to a change in a file, do
% when /path/to/file command