Missing answers to questions

[iddm]

The repository contains invalid link to 020.md answer and also 021.md is missing. Is it intentional?

From Wikipedia Tail Call

 foo:
  call B
  call A
  ret

Tail call elimination replaces the last two lines with a single jump instruction:

 foo:
  call B
  jmp  A

After subroutine A completes, it will then return directly to the return address of foo, omitting the unnecessary ret statement.