Filtering Associative Array Key-Values the D way

[David DeWitt via Digitalmars-d-learn]

I'm trying to understand filtering an Associative Array the D 
way.  I have the code below
(Using while readln cause problem failing on Debian using 
byLineCopy()).  When the byKeyValue().filter evaluates to reduce 
the number of Keys:Values to only the ones in the filtered 
header, what is the best way to transform the Pair back to an AA 
instead of having to run the foreach loop after?  Basically if 
the file has 300 columns and 300 in the full header but only 15 
in the filtered header I want the return of rec to only be the 15 
in an Associative Array.  Thanks :)

    string line;
    string[string][] records;
    while ((line = f.readln()) !is null){
        string[string] record;
        auto rec = assocArray(zip(h1.fullHeader, 
splitter(line,',')))
        
.byKeyValue().filter!(a=>h1.filteredHeader.canFind(a.key));

        foreach(r;rec){
            record[r.key] = r.value;
        }
        records ~= record;
    }