* @-patterns as in Haskell. For instance, in a function definition

f = args @ {x, y, z}: ...;

  `args' refers to the argument as a whole, which is further
  pattern-matched against the attribute set pattern {x, y, z}.

src/libexpr/nixexpr-ast.def

@@ -77,6 +77,7 @@ Scope | | Expr |
 
 VarPat | string | Pattern |
 AttrsPat | ATermList | Pattern |
+AtPat | Pattern Pattern | Pattern |
 
 Formal | string DefaultValue | ATerm |