syntax definition with only the head part works

Stream: lean4

[Locria Cyber]

structure Tagged (tag : String) (a : Type u) where
  inner: a

def ABC := Tagged "a" String ⊕ Tagged "b" String ⊕ Tagged "c" String

open Lean
syntax "tagged": term
-- syntax "tagged" str ident: term

macro "tagged " tag:str val:ident : term => do
  let rec build_pattern : Nat → Term → MacroM Term
    | 0, acc => `(Sum.inl $acc)
    | n+1, acc => do `(Sum.inr $(← build_pattern n acc))
  -- This is a simplified example assuming tags "a", "b", "c" correspond to positions 0, 1, 2
  let position := match tag.getString with
    | "a" => 0
    | "b" => 1
    | "c" => 2
    | _ => panic! "Unsupported tag"
  build_pattern position (← `(⟨$val⟩))

def process (abc : ABC) :=
  match abc with
  -- | .inl ⟨x⟩ => x
  | tagged "a" x => x
  | .inr (.inl ⟨x⟩) => x
  | .inr (.inr ⟨x⟩) => x

I'm curious why syntax "tagged" str ident: term doesn't work?

[Sebastian Ullrich]

macro already creates syntax