{
    a simple key: a value,  # Note that the KEY token is produced.
    ? a complex key: another value,
}
# STREAM_START                                  FYTT_STREAM_START
# FLOW_MAPPING_START                            FYTT_FLOW_MAPPING_START
# KEY                                           FYTT_KEY
# SCALAR value='a simple key' style=PLAIN       FYTT_SCALAR value='a simple key' style=PLAIN
# VALUE                                         FYTT_VALUE
# SCALAR value='a value' style=PLAIN            FYTT_SCALAR value='a value' style=PLAIN
# FLOW_ENTRY                                    FYTT_FLOW_ENTRY
# KEY                                           FYTT_KEY
# SCALAR value='a complex key' style=PLAIN      FYTT_SCALAR value='a complex key' style=PLAIN
# VALUE                                         FYTT_VALUE
# SCALAR value='another value' style=PLAIN      FYTT_SCALAR value='another value' style=PLAIN
# FLOW_ENTRY                                    FYTT_FLOW_ENTRY
# FLOW_MAPPING_END                              FYTT_FLOW_MAPPING_END
# STREAM_END                                    FYTT_STREAM_END