std/tables [] vs. add()

[mildred]

When replacing `Table[string,string]` with `Table[string,seq[string]]` how do 
you efficiently add an item to the table?

I came up with:
    
    
    result[key] = concat(result.getOrDefault(key), @[value])
    
    
    Run

But it requires a lookup, a concat and a store. Is there a better way? I tried 
`result[key].add(value) but it obviously does not work`