-- | Retrieves all predicates from a text
prd_text :: TEXT -> Set.Set NAME
prd_text t =
    case t of
         Text ps _ -> prd_phrases ps
         Named_text _ nt _ -> prd_text nt
prd_text :: TEXT -> Set.Set NAME
prd_text t =
    case t of
         Text ps _ -> prd_phrases ps
         Named_text _ nt _ -> prd_text nt
prd_text t =
    case t of
         Text ps _ -> prd_phrases ps
         Named_text _ nt _ -> prd_text nt