doc-src/Tutorial/CodeGen/expr

error msg: counting from one (again), in order to be consistent with
case names of induction rule;

types 'v binop = 'v => 'v => 'v
datatype ('a,'v) expr = Cex 'v
                      | Vex 'a
                      | Bex ('v binop)  (('a,'v) expr) (('a,'v) expr)