anchor.diff:  %% catch
   %%------------------------------------------------------------------------------
   ast(#{op := 'catch'} = Expr, State) ->
  +  ?DEBUG('catch'),
     #{ class := ErrType
      , local := PatternExpr
      , body  := BodyExpr
  
positive.code: %% methods
  %%------------------------------------------------------------------------------
  ast(#{op := fn_method} = Expr, State0) ->
    #{ name := Name
     , env  := Env
     } = Expr,
  
negative.code: %% letfn
  %%------------------------------------------------------------------------------
  ast(#{op := letfn} = Expr, State) ->
    ?DEBUG(letfn),
    #{ vars := VarsExprs
     , fns  := FnsExprs
     , body := BodyExpr
  