// A grammar is a set of production rules for strings in a given
  // languages. In this case, the grammar is a function from a
  // character to its successor, and is defined inductively through
  // some optimization process.
  type IGrammar =
    abstract member Continue: string -> string
    abstract member Entropy: string -> float
    abstract member Mutate: int -> unit
    abstract member Clone: unit -> IGrammar
    abstract member Compress: unit -> int list