-- % opentag(b) --> [\[], [b], [\]]
-- % opentag(u) --> [\[], [u], [\]]
-- % opentag(o) --> [\[], [o], [\]]
-- % opentag(i) --> [\[], [i], [\]]
opentagRule i = [ Rule 0 (Functor "b" []) [ Left "[b]" ]
                , Rule 0 (Functor "u" []) [ Left "[u]" ]
                , Rule 0 (Functor "o" []) [ Left "[o]" ]
                , Rule 0 (Functor "i" []) [ Left "[i]" ]
                ]

-- % closetag(b) --> [\[], [/], [b], [\]]
-- % closetag(u) --> [\[], [/], [u], [\]]
-- % closetag(i) --> [\[], [/], [i], [\]]
closetagRule i = [ Rule 0 (Functor "b" []) [ Left "[/b]" ]
                 , Rule 0 (Functor "u" []) [ Left "[/u]" ]
                 , Rule 0 (Functor "o" []) [ Left "[/o]" ]
                 , Rule 0 (Functor "i" []) [ Left "[/i]" ]
                 ]

-- % bbtag(tag(T,B)) --> opentag(T), bbcode(B), closetag(T)
-- % bbtag(str(S)) --> #string without [#
bbtagRule i = [ Rule 2 (Functor "tag" [ Var (i + 0), Var (i + 1) ]) [ Right (opentagRule, Var (0 + i)), Right (bbcodeRule, Var (1 + i)), Right (closetagRule, Var (0 + i)) ]
              , RuleStrExcept "[" True
              ]

-- % bbcode(bb(B,BS)) --> bbtag(B), bbcode(BS)
-- % bbcode(eps) --> []
bbcodeRule i = [ Rule 2 (Functor "bb" [ Var (i + 0), Var (i + 1) ]) [ Right (bbtagRule, Var (0 + i)), Right (bbcodeRule, (Var (1 + i))) ]
               , Rule 0 (Functor "eps" []) []
               ]