doc-src/Tutorial/CodeGen/expr

'oops' made proper;
removed 'kill_proof' (superceded by 'kill');

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