From [Rodney Huddleston Geoffrey K Pullum. (2017). The Cambridge Grammar of the English Language. p.23.]
We can represent the structure of sentences in more detail than is done in a diagram like [2] if we show the category to which each constituent belongs, as in [6].
Should it be detail(than detail is represented in a diagram), or the structure(than the structure is represented in a diagram)?
Syntactically, the detail is more appropriate,but,semantically,the structure is more appropriate.
Consider someone talking about how to make a painting of a fish:
We can render (the appearance of the fish) ( in a more realistic style) than was done on this painting to my left, if we apply some silver paint.
With "than was done", the comparand must be the entire verb phrase "render the appearance of the fish in a more realistic style". do is semantically anchored to the verb.
If we choose a different verb, for example, "than was used" or "than was employed", the comparand becomes "style". was employed is anchored to what was employed, the style.
We can render (the appearance of the fish) ( in a more realistic style) than was employed on this painting to my left, if we apply some silver paint.
I personally find the "done" version somewhat awkward, as the semantic focus is on the style chosen to render a realistic appearance, not the action of rendering a realistic appearance per se.
The basic structure is "We can (action) (comparison) than (method1) if we (method2)", so:
We can represent the structure of sentences in more detail than is done in a diagram like figure 2 if we show the category to which each constituent belongs.
Is broken down as
The clause "in more detail", as a whole, qualifies the action "represent the structure of sentences", and compares two methods of achieving that.
Grammatically, the comparison could be replaced, like "using fewer colours", or even just "better".
Compare a much simpler example:
We can move ourselves faster than by walking if we run
Sample sentence:
We can represent the structure of sentences in more detail than is done in a diagram like [2] if we show the category to which each constituent belongs, as in [6].
Answer to the OP's question: Nothing is omitted in "represent the structure of sentences in more detail than is done in a diagram like [2] if we show the category to which each constituent belongs, as in [6]."
Possible synonyms for done if one must use one:
We can represent the structure of sentences in more detail than is done in a diagram like [2] if we show the category to which each constituent belongs, as in [6].
to do something in more detail than is done, used or shown in [some thing] is idiomatic in English.
