block_node_or_indentless_sequence ::= ALIAS
| properties (block_content | indentless_block_sequence)?
| block_content
| indentless_block_sequence
block_node ::= ALIAS
| properties block_content?
| block_content
flow_node ::= ALIAS
| properties flow_content?
| flow_content
properties ::= TAG ANCHOR? | ANCHOR TAG?
block_content ::= block_collection | flow_collection | SCALAR
flow_content ::= flow_collection | SCALAR
block_collection ::= block_sequence | block_mapping
flow_collection ::= flow_sequence | flow_mapping