I have a function collect extra keyword arguments using `...`, so it is like `function f(args=0; kwargs...)`. I want to check if a keyword argument, let' say, `a`, exists in `kwargs`. 

What I do probably is not an elegant way, I first create a `Dict` to store the keywords and corresponding values `kwargs_dict=[key=>value for (key, value) in kwargs]`, then I use `haskey(kwargs_dict, :a)` to check if `a` is a key in the dict. Then I get its value by `kwargs_dict[:a]`.

```
function f(; kwargs...)
   kwargs_dict = [key=>value for (key, value) in kwargs]
   haskey(kwargs_dict, :a)
   a_value = kwargs_dict[:a]
end

f(args=0, a=2)
> true

f(args=0)
> false

```

I wonder if there is better way to check if the keyword argument `a` is in `kwargs` and to get the value of the existed keyword argument.