fix the "seq expr" print placeholder

chapter1/chapter1.sml

@@ -86,7 +86,7 @@ and expAsStr t (NumExp x)      = Int.toString x
     in
         Int.toString v1
     end
-  | expAsStr t (EseqExp (s,e)) = "seq expr"
+  | expAsStr t (EseqExp (s,e)) = expAsStr (interpStm t s) e
 
 
 fun interp s = interpStm [] s