code: def find_enclosing_bracket_left(self, left_ch, right_ch, start_pos=None):
          
          if self.current_char == left_ch:
              return 0
  
          if start_pos is None:
              start_pos = 0
          else:
              start_pos = max(0, start_pos)
  
          stack = 1
  
          # Look backward.
          for i in range(self.cursor_position - 1, start_pos - 1, -1):
              c = self.text[i]
  
              if c == right_ch:
                  stack += 1
              elif c == left_ch:
                  stack -= 1
  
              if stack == 0:
                  return i - self.cursor_position
docs: Find the left bracket enclosing current position. Return the relative
          position to the cursor position.
  
          When `start_pos` is given, don't look past the position.