0 vs 1 indexing has an actual semantic difference that you have to keep in your head at ~all times, but could have a text editor plugin that rewrites func<->fn since it's purely syntactical.

What is inconvenient about it?

fun is the best, as it makes the function name align with the code block indented 4 characters :D

    fun main() {
        println("Hello, world!")
    }
        ^ aligned here! \o/

fun

Usually as "function"

fn