Greetings to @Morozzzko, the Ruby is neat.

README.md

Hi folks here I show some Ruby magic.

• a.group_by{|x| x} is similar to a.group_by(&:itself)
• a.group_by(&:itself) is a short cut for a.group_by(&:itself.to_proc)
• and a.group_by(&:itself.to_proc) just use new Proc a.group_by(&Proc.new{|o| o.send :itself})
• a.group_by(&Proc.new{|o| o.send :itself}) use & to treat a Proc as block like this a.group_by{|o| o.send :itself}
• a.group_by{|o| o.send :itself} here it's possible to substitute o.send :itself to o.itself: a.gruop_by{|o| o.itself}
• a.group_by{|o| o.itself} here o.itself it's just an o so it's equal to a.group_by{|o| o}
• Hey and the magic is a.group_by{|o| o} is just a a.group_by{|x| x}

to_proc.rb

# https://stackoverflow.com/a/12725405/8574922
# https://apidock.com/ruby/Enumerable/group_by
p [1,1,1,1,1,2,2,2,15,15,20,25,25,40,4,4].group_by{|x| x}.values
p [1,1,1,1,1,2,2,2,15,15,20,25,25,40,4,4].group_by(&:itself).values # @morozzzko
p [1,1,1,1,1,2,2,2,15,15,20,25,25,40,4,4].group_by(&:itself.to_proc).values
p [1,1,1,1,1,2,2,2,15,15,20,25,25,40,4,4].group_by(&Proc.new{|o| o.send :itself}).values
p [1,1,1,1,1,2,2,2,15,15,20,25,25,40,4,4].group_by{|o| o.send :itself}.values
p [1,1,1,1,1,2,2,2,15,15,20,25,25,40,4,4].group_by{|o| o.itself}.values  
p [1,1,1,1,1,2,2,2,15,15,20,25,25,40,4,4].group_by{|o| o }.values
# https://goo.gl/yyYoHD