rule:
  - non_terminal
  - terminal
non_terminal:
  - [ a_rule  ]
  - [ a_rule+ ]
  - [ a_rule* ]
  - [ "a_rule?" ]
  - [ a_rule, a_rule, a_rule ]
a_rule:
  - [ terminal, non_terminal ]
terminal:
  - regexp_nt
  - true_nt
  - false_nt
  - nil_nt
regexp_nt:
  !ruby/regexp /^[a-z]+$/
true_nt:
  true
false_nt:
  false
nil_nt:
  ~