code: def replace_line_magic(source, magic, template='{line}'):
      
      filtered = []
      for line in source.splitlines():
          if line.strip().startswith(magic):
              substitution = template.format(line=line.replace(magic, ''))
              filtered.append(substitution)
          else:
              filtered.append(line)
      return '\n'.join(filtered)
docs: Given a cell's source, replace line magics using a formatting
      template, where {line} is the string that follows the magic.