Ruby

Both RDoc and Rurema have descriptions of Enumerable#slice_before with this example:

# split mails in mbox (slice before Unix From line after an empty line) open("mbox") { |f| f.slice_before(emp: true) { |line, h| prevemp = h[:emp] h[:emp] = line == "\n" prevemp && line.start_with?("From ") }.each { |mail| mail.pop if mail.last == "\n" pp mail } } 

which doesn't work.

zsh % ruby -v tmp.rb ruby 2.5.0dev (2017-02-02 trunk 57498) [x86_64-darwin15] tmp.rb:3:in `slice_before': wrong number of arguments (given 1, expected 0) (ArgumentError) from tmp.rb:3:in `block in <main>' from tmp.rb:2:in `open' from tmp.rb:2:in `<main>'