# The following YAML grammar is LL(1) and is parsed by a recursive descent parser.

 stream            ::= STREAM-START implicit_document? explicit_document* STREAM-END
 implicit_document ::= block_node DOCUMENT-END*
 explicit_document ::= DIRECTIVE* DOCUMENT-START block_node? DOCUMENT-END*
 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
 block_sequence    ::= BLOCK-SEQUENCE-START (BLOCK-ENTRY block_node?)* BLOCK-END
 indentless_sequence   ::= (BLOCK-ENTRY block_node?)+
 block_mapping     ::= BLOCK-MAPPING_START
                       ((KEY block_node_or_indentless_sequence?)?
                       (VALUE block_node_or_indentless_sequence?)?)*
                       BLOCK-END
 flow_sequence     ::= FLOW-SEQUENCE-START
                       (flow_sequence_entry FLOW-ENTRY)*
                       flow_sequence_entry?
                       FLOW-SEQUENCE-END
 flow_sequence_entry   ::= flow_node | KEY flow_node? (VALUE flow_node?)?
 flow_mapping      ::= FLOW-MAPPING-START
                       (flow_mapping_entry FLOW-ENTRY)*
                       flow_mapping_entry?
                       FLOW-MAPPING-END