nil
                                     ^
                                     | parent
 +-----------------------------------------------+
 |   def main                                    |
 |     a = "Hello"                               |
 |                              ^                |
 |                              | parent         |
 |    +-----------------------------------+      |
 |    | [1, 2, 3].each do |b|             |      |
 |    |                         ^         |      |
 |    |                         | parent  |      |
 |    |  +-------------------------+      |      |
 |    |  | say_hello do            |      |      |
 |    |  |   puts "#{a}: #{b}"     |      |      |
 |    |  | end                     |      |      |
 |    |  +-------------------------+      |      |
 |    | end                               |      |
 |    +-----------------------------------+      |
 |   end                                         |
 +-----------------------------------------------+