@native("Module", "function", dep1, dep2, ...): a rule body that delegates to hand-written Elixir code (an Ichor.CustomRule implementation) instead of ordinary Grammar.IR combinators -- the escape hatch for constructs no static PEG grammar can express (a Prolog-style mutable operator-precedence table, C's typedef-vs- expression ambiguity, and similar "what matches here depends on something read earlier" problems).

deps names the only other rules module/function is allowed to call back into (via the rule_matchers map Ichor.CustomRule.match/4 receives) -- keeping this node's dependencies visible to Grammar.Analysis's reference check even though its own body is opaque. nullable/leading are author-supplied facts standing in for what Grammar.Analysis would otherwise compute structurally (it can't, since the body is arbitrary code): nullable defaults to false (matches the common case -- most custom recognizers consume something), leading defaults to deps (conservative -- assumes it might call any declared dependency as its very first, unconsumed action, for left-recursion-cycle detection).