Preprocessing to strip tags in SGM files.

def _preprocess_sgm(line, is_sgm):
  """Preprocessing to strip tags in SGM files."""
  if not is_sgm:
    return line
  # In SGM files, remove <srcset ...>, <p>, <doc ...> lines.
  if line.startswith("<srcset") or line.startswith("</srcset"):
    return ""
  if line.startswith("<doc") or line.startswith("</doc"):
    return ""
  if line.startswith("<p>") or line.startswith("</p>"):
    return ""
  # Strip <seg> tags.
  line = line.strip()
  if line.startswith("<seg") and line.endswith("</seg>"):
    i = line.index(">")
    return line[i + 1:-6]