[Agda] when to set underscore suffix?

[mechvel at scico.botik.ru]

On 2020-11-23 21:51, Georgi Lyubenov wrote:
> That's what I'm referring to, yes. For more examples of usage you can
> check Part 2 of https://plfa.github.io/
> 

I do not find there any explanation on the subject.
And I suspect that every unary function is going to be combined with 
operators.
For example,
     length : List A -> Nat,

is combined with  _++_, _+_, _≈_
and is likely to be combined with  _*_, +_.
Still it has not underscore.
May be underscore for the function name  foo  is used when the author is 
eager for
more possibility to skip brackets in expressions with  foo
(?)

Regards,

------
Sergei