docs: def get_indented_block(prefix_lines):
      """Returns an integer.
  
      The return value is the number of lines that belong to block begun
      on the first line.
  
      Parameters
      ----------
  
        prefix_lines : list of basestring pairs
          Each pair corresponds to a line of SHPAML source code. The
          first element of each pair is indentation. The second is the
          remaining part of the line, except for trailing newline.
      """
      prefix, line = prefix_lines[0]
      len_prefix = len(prefix)
  
      # Find the first nonempty line with len(prefix) <= len(prefix)
      i = 1
      while i < len(prefix_lines):
          new_prefix, line = prefix_lines[i]
          if line and len(new_prefix) <= len_prefix:
              break
          i += 1
  
      # Rewind to exclude empty lines
      while i - 1 > 0 and prefix_lines[i - 1][1] == '':
          i -= 1
  
      return i