data Ast
  | Const   i   :: Int
  | Plus    l,r :: Ast
  | Def     x   :: String  e :: Ast
  | Use     x   :: String


itf Analyze
  inh ast :: Ast
  tail dfr :: Defer

itf Defer
  tail nxt :: Sum

itf Sum
  syn v   :: Int


{
sast = sem :: Analyze
         clause Const
           loc@Ast.Const = lhs.ast
           tail sem :: Sum of
                  lhs.v = loc.i

         clause Plus
           loc@Ast.Plus = lhs.ast
           child l :: Sum = loc.l
           child r :: Sum = loc.r
           visit r
           l.ast = loc.l
           r.ast = loc.r
           
           tail sem :: Sum of
                  visit l.nxt
                  visit r.nxt
                  lhs.v = l.v + r.v

         clause Def
           loc@Ast.Def = lhs.ast
           child e :: Sum = loc.e
           visit e
           e.ast = loc.e
           
           tail sem :: Sum of
                  visit e.nxt
                  lhs.v = e.v

         clause Use
           loc@Ast.Use = lhs.ast
           tail sem :: Sum of
                  lhs.v = ...?
}