code: func (self *FileSet) AddFile(filename, src string) int {
  	base := self.nextBase()
  	file := &File{
  		name: filename,
  		src:  src,
  		base: base,
  	}
  	self.files = append(self.files, file)
  	self.last = file
  	return base
  }
docs: // AddFile adds a new file with the given filename and src.
  //
  // This an internal method, but exported for cross-package use.