sequence_push
sequence_pop
group_push
group_pop
mapping_start {data}
mapping_end
named_mapping {text} {data}
numbered_mapping {number} {data}
comment {data}
document_start {text-type}
document_end
date {date-type} x
type {text-type}
define_alias {text-ref} {data}
alias {text-ref}

need to recognise
- nested sequences
- nested mappings
