<PsiElement.firstChild vs PsiElement.children.firs...

# stackoverflow

[user]

PsiElement.firstChild vs PsiElement.children.first()
I expected PsiElement.firstChild and PsiElement.children.first() to always return the same PsiElement, however, it turns out that (considering KtElements):
for KtBlockExpression they are not the same: firstChild returns a left curly brace and children.first() returns the first expression of the body.
for KtTypeReference they are the same thing.
So the questions is:
Why are they not equivalent in general?
When are the equivalent and when are they different (I am interested mainly in classes...