In-class collaborative exercises.  Extend the Java-subset syntax with the
following productions.  Indicate the changes to the java1.ss Java type
checker to incorporate each change.


1. MultiplicativeExpression -> MultiplicativeExpression % CastExpression

   (If you understand java1.ss, this is very easy: 2 lines!)


2. Statement -> IterationStatement

   IterationStatement -> while ( Expression ) Statement

   (This is pretty easy: about 6 lines.)


3. Type Declaration -> InterfaceDeclaration

   ClassDeclaration -> ClassModifiers class Identifier Interfaces ClassBody

   InterfaceDeclaration -> interface Identifier Interfaces InterfaceBody

   InterfaceDeclaration -> interface Identifier InterfaceBody

   InterfaceBody -> { FieldDeclarations }

   Interfaces -> implements TypeNameList

   TypeNameList -> TypeName ,*
   
   (This is hard.)