| While (b0,c0) ->
        if (eval_bexp b0 sigma) then
            begin match (eval_com c0 sigma) with
                | Normal (sigma') -> eval_com (While(b0, c0)) sigma'
                | Exceptional(_,_) as e -> e
            end
        else
            Normal(sigma)
| TryCatch(c1,x,c2) ->
            begin match eval_com c1 sigma with
            | Normal(sigma') -> Normal(sigma')
            | Exceptional (sigma', n) -> eval_com c2 (update sigma' x n)
            end