(tree-transform-if predicate
                       transformer
                       (second tree)
                       depth)

> Nineteen! Nineteen spaces of indentation! It's getting unruly.

Why, is there a shortage of whitespace these days??

> Such an indent, when used in deeply nested code, makes it too wide and unreadable.

Ah, I see! Well, I would argue the problem is not the indentation style. What makes your code unreadable is that it's too deply nested, and I would bet that no indentation style can help with that.