Interpreter for Commands.

Staging the interpreter for commands proceeds in a similar manner:

(*  interpret2 : Com -> index ->  <unit M> *)
fun interpret2 stmt index =
case stmt of
  Assign(name,e) =>
   let val loc = position name index
   in <Do mswo { n <- ~(eval2 e index) ;
                 write ~(lift loc) n }>
   end
| Seq(s1,s2) =>
   <Do mswo { x <- ~(interpret2 s1 index);
              y <- ~(interpret2 s2 index);
              Return mswo () }>
| Cond(e,s1,s2) =>
   <Do mswo 
       { x <- ~(eval2 e index);
         if x=1
            then ~(interpret2 s1 index)
            else ~(interpret2 s2 index)}>
| While(e,b) =>
   <let fun loop () =
        Do mswo 
        { v <- ~(eval2 e index);
          if v=0
             then Return mswo ()
             else Do mswo 
                  { q <- ~(interpret2 b index); 
                    loop ()}
        }
    in loop () end>
| Declare(nm,e,stmt) =>
   <Do mswo { x <- ~(eval2 e index) ;
              push x ;
              ~(interpret2 stmt (nm::index)) ;
              pop }>
| Print e =>
   <Do mswo { x <- ~(eval2 e index) ;
              output x }>;