Get a Symbol vector of all variable names in an Expr?

General Usage
2

It’s pretty easy to walk expression trees, typing :(x^2+1) |> dump etc. will let you see how they are structured. Here’s a rough solution, but there are expressions which will trip this up:

julia> walk!(list) = ex -> begin
       ex isa Symbol && push!(list, ex)
       ex isa Expr && ex.head == :call && map(walk!(list), ex.args[2:end])
       list
       end
walk! (generic function with 1 method)

julia> :(x^2+sum(x,y-3,z)) |> walk!([])
4-element Array{Any,1}:
 :x
 :x
 :y
 :z
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