Provide call stacks for empty & <|> assignments.

src/Data/Syntax/Assignment.hs

@@ -1,4 +1,4 @@
-{-# LANGUAGE DataKinds, GADTs, ScopedTypeVariables, StandaloneDeriving, TypeFamilies #-}
+{-# LANGUAGE DataKinds, GADTs, InstanceSigs, ScopedTypeVariables, StandaloneDeriving, TypeFamilies #-}
 -- | Assignment of AST onto some other structure (typically terms).
 --
 --   Parsing yields an AST represented as a Rose tree labelled with symbols in the language’s grammar and source locations (byte Range and SourceSpan). An Assignment represents a (partial) map from AST nodes onto some other structure; in essence, it’s a parser that operates over trees. (For our purposes, this structure is typically Terms annotated with source locations.) Assignments are able to match based on symbol, sequence, and hierarchy; thus, in @x = y@, both @x@ and @y@ might have the same symbol, @Identifier@, the left can be assigned to a variable declaration, while the right can be assigned to a variable reference.
@@ -242,7 +242,9 @@ makeState source nodes = AssignmentState 0 (Info.SourcePos 1 1) source nodes
 -- Instances
 
 instance Enum symbol => Alternative (Assignment (Node symbol)) where
+  empty :: HasCallStack => Assignment (Node symbol) a
   empty = Empty `Then` return
+  (<|>) :: HasCallStack => Assignment (Node symbol) a -> Assignment (Node symbol) a -> Assignment (Node symbol) a
   a <|> b = case (a, b) of
     (_, Empty `Then` _) -> a
     (Empty `Then` _, _) -> b