When projecting a non-existent field, return Prelude.False

This seems to be GF's own behaviour, as exhibited by the canonical version of PhrasebookTha:

    NNumeral Numeral_0 = {s = Numeral_0.s; hasC = <>.hasC};

src/compiler/GF/Compile/GrammarToLPGF.hs

@@ -157,7 +157,11 @@ mkCanon2lpgf opts gr am = do
                 -- find label index in argument type
                 let Just (C.RecordType rrs) = mtyp
                 let rrs' = [ lid | C.RecordRow lid _ <- rrs ]
-                lblIx <- eitherElemIndex lblId rrs'
+                -- lblIx <- eitherElemIndex lblId rrs'
+                let
+                  lblIx = case eitherElemIndex lblId rrs' of
+                    Right x -> x
+                    Left _ -> 0 -- corresponds to Prelude.False
                 -- lookup lintype for record row
                 let C.RecordRow _ lt = rrs !! lblIx
                 return (L.LFProjection v1' (L.LFInt (lblIx+1)), Just lt)