code: def parse_block_mapping_first_key(self):
      token = self.get_token()
      self.marks.append(token.start_mark)
      return self.parse_block_mapping_key()
  
imports: from .error import MarkedYAMLError
  from .tokens import *
  from .events import *
  from .scanner import *
label_desc: Use image node_id 19 for calling the Parser obj's underlying member method code with example usage: obj.parse_block_mapping_first_key() and returns: self