doc-src/Tutorial/CodeGen/expr

added inform_file_processed, inform_file_retracted;
proper outer syntax setup;
more robust theory loader actions;

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