sk():
  /[sk]/ |
  /`[sk]/ + sk() |
  /``s/ + sk() + sk() |
  {/``k/; a <- sk(); sk(); a} |
  {/```s/; a <- sk(); b <- sk(); c <- sk(); sk["``" + a + c + "`" + b + c]} |
  {/```k/; a <- sk(); sk(); b <- sk(); sk["`" + a + b]} |
  {/````/; a <- sk(); b <- sk(); c <- sk(); d <- sk(); e <- sk();
   sk["`" + sk["```" + a + b + c + d] + e]}.