in your example ``` val newLines = markdown.preToc...

# coroutines

[vaskir]

in your example

val newLines = markdown.preTocText +
            (if (toc.isEmpty()) emptyList<String>() else emptyList<String>() + "" + toc + "") +
            markdown.postTocText

list literals could improve readability a lot:

val newLines = markdown.preTocText +
            (if (toc.isEmpty()) [] else ["", toc, ""] + markdown.postTocText