Indent specific individual paragraphs of a UITextView

Question

I'm building up the textual content of a UITextView, by adding individual paragraphs one at a time to an NSMutableAttributedString. Some of these paragraphs need to be indented, as they are hierarchical, as in:

1. ...    
   (a). ... 
       (i). ...
       (ii). ...
   (b). ...
...

To (try to) do the indentation, I'm using this code:

let paraStyle = NSMutableParagraphStyle()
paraStyle.firstLineHeadIndent = CGFloat(indentLevel * 20)
paraStyle.headIndent = CGFloat(indentLevel * 20) 
attrText.addAttribute(.paragraphStyle, value: paraStyle, range:NSRange(location: 0, length: attrText.length))

and finally

textView.attributedText = attrText

where indentLevel indicates how much a given paragraph is to be indented.

When I look at the final attributed text in the debugger, I can see the correct paragraph styling is in place, but the text on-screen is NOT indented at all; everything is left-justified.

What am I missing/misunderstanding about styling paragraphs using attributed strings in a UITextView?