code: func (self *FileSet) Position(idx Idx) *Position {
  	for _, file := range self.files {
  		if idx <= Idx(file.base+len(file.src)) {
  			return file.Position(idx - Idx(file.base))
  		}
  	}
  
  	return nil
  }
docs: // Position converts an Idx in the FileSet into a Position.