do-copy

Called to copy a region of the editor into the clipboard. This method is provided so that it can be overriden by subclasses. Do not call this method directly; instead, call copy.

• (send a-text do-copy start end time extend?) -> void
      start : exact non-negative integer
      end : exact non-negative integer
      time : exact integer
      extend? : boolean

  Copy the data from start to end, extending the current clipboard contexnts if extend? is not #f.

  See Cut and Paste for a discussion of the time argument. If time is outside the platform-specific range of times, an exn:application:mismatch exception is raised.