an executable by-name without forking or using externals.
In a performance benchmark of 10,000 runs on circa 2006 hardware, f_which
out-performed `which' with an average completion time of ~2.5 seconds versus
~56 seconds.
This should be handy for future use (not that I make it a habit to call
`which' in a loop 10,000 times).
c0adcdb9c1