# Prefixing lines with line numbers:
linesAndNumbers = for line, linenumber in file.read "original"
	yield "#{linenumber}: #{line}"

for lineAndNumber in linesAndNumbers
	console.log lineAndNumber

# do some funky work, file "original" is being modified

# run the code again.
for lineAndNumber in linesAndNumbers
	console.log lineAndNumber