Why can't I borrow slice assignment for distinct strings?

[benjamindlee]

As a follow up for potential future readers, this does work:
    
    
    proc `$`*(x: Dna): string {.borrow.}
    proc `[]=`*[T, U](s: var Dna, x: HSlice[T, U], b: Dna) =
      var tmp = $s
      s[x] = b
      s = tmp.Dna
    
    
    Run

This seems inefficient but does work around the problem.