pp-aterm does not print annotations on strings

$ pp-aterm
SDefT("test1_0_0",[],[],Scope(["a_0"{"escapes"},"b_0"{"escapes"}],Let([
SDefT("c_0",[],[],Match(Anno(Op("",[Var("b_0"{("frames",["c_0"])}){"(un)bound"},
Var("a_0"{("frames",["c_0"])}){"(un)bound"}]),Wld))){(0,0)},SDefT("d_0",[],[],Build(Anno(Op("",[Var("a_0"{("frames",["d_0"])}){"(un)bound"},Var("b_0"{("frames",
["d_0"])}){"(un)bound"}]),Op("Nil",[])))){(0,0)}],CallT(SVar("seq_2_0"{("frames"
,["test1_0_0"])}),[CallT(SVar("c_0"{("frames",[])}),[],[]),CallT(SVar("d_0"{("frames",[])}),[],[])],[])))){(0,0)}

produces

SDefT(
  "test1_0_0"
, []
, []
, Scope(
    ["a_0", "b_0"]
  , Let(
      [ SDefT(
          "c_0"
        , []
        , []
        , Match(
            Anno(
              Op("", [Var("b_0"){"(un)bound"}, Var("a_0"){"(un)bound"}])
            , Wld
            )
          )
        ){(0, 0)}
      , SDefT(
          "d_0"
        , []
        , []
        , Build(
            Anno(
              Op("", [Var("a_0"){"(un)bound"}, Var("b_0"){"(un)bound"}])
            , Op("Nil", [])
            )
          )
        ){(0, 0)}
      ]
    , CallT(
        SVar("seq_2_0")
      , [ CallT(SVar("c_0"), [], [])
        , CallT(SVar("d_0"), [], [])
        ]
      , []
      )
    )
  )
){(0, 0)}