block.rb

def hello str, &block
  pre_str = str + " world!"
  block.call pre_str
  after_str = "say " + str
  puts after_str
end

hello "hello" do |str|
  puts "MC says #{str}"
end

hello "hello" do |str|
  puts "YL says #{str}"
end

##### outputs

# MC says hello world!
# say hello
# YL says hello world!
# say hello