query - Unix-like reference to parent and root Association?

Is there a syntax or function to reference parent and root when querying nested Associations similar to Unix' ".." and "/", eg "cd ../../mydir". ?

Here's a typical application of the root construct. Given a nested dataset:

ds = <|  "study" -> <|   "all"  -> <|"n" -> 223.`, "Median" -> 18.5` |>|>, 
        "bySite" -> <|   "VASD" -> <|"n" -> 107.`, "Median" -> 20.40`|>, 
                         "UCSD" -> <|"n" -> 116.`, "Median" -> 16.5` |>|>, 
  "bySpeciality" -> <|"Primary" -> <|"n" -> 127.`, "Median" -> 17.8` |>, 

                    "Specialty" -> <|"n" -> 96.` , "Median" -> 19.5` |>|>|> // Dataset

(Dataset is only used here for formatting convenience)

using ds["study","all","n"] as denominator, the query is to convert all values of the instances "n" to ratios:

   ds[All, All, #/ds["study", "all", "n"] &]

But how can it be used in a query without reference to ds? For example, would like to use something like RootAssociation

ds // Query[All, All, {"n" -> (#/RootAssociation["study", "all", "n"] &)}]

ParentAssociation would work in a similar way

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functions - Get leading series expansion term?

Given a function f[x] , I would like to have a function leadingSeries that returns just the leading term in the series around x=0 . For example: leadingSeries[(1/x + 2)/(4 + 1/x^2 + x)] x and leadingSeries[(1/x + 2 + (1 - 1/x^3)/4)/(4 + x)] -(1/(16 x^3)) Is there such a function in Mathematica? Or maybe one can implement it efficiently? EDIT I finally went with the following implementation, based on Carl Woll 's answer: lds[ex_,x_]:=( (ex/.x->(x+O[x]^2))/.SeriesData[U_,Z_,L_List,Mi_,Ma_,De_]:>SeriesData[U,Z,{L[[1]]},Mi,Mi+1,De]//Quiet//Normal) The advantage is, that this one also properly works with functions whose leading term is a constant: lds[Exp[x],x] 1 Answer Update 1 Updated to eliminate SeriesData and to not return additional terms Perhaps you could use: leadingSeries[expr_, x_] := Normal[expr /. x->(x+O[x]^2) /. a_List :> Take[a, 1]] Then for your examples: leadingSeries[(1/x + 2)/(4 + 1/x^2 + x), x] leadingSeries[Exp[x], x] leadingSeries[(1/x + 2 + (1 - 1/x...

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