https://github.com/ImageMagick/ImageMagick6/issues/43
MagickExport void SetCustomStreamSeeker(CustomStreamInfo *custom_stream,
  CustomStreamSeeker seeker)
{
  assert(custom_stream != (CustomStreamInfo *) NULL);
  assert(custom_stream->signature == MagickCoreSignature);
  custom_stream->seeker=seeker;
}

MagickExport void SetCustomStreamSeeker(CustomStreamInfo *custom_stream,
  CustomStreamSeeker seeker)
{
  assert(custom_stream != (CustomStreamInfo *) NULL);
  assert(custom_stream->signature == MagickCoreSignature);
  custom_stream->seeker=seeker;
}
