doc-src/Tutorial/CodeGen/comp

Added an exception handler and error msg.

consts comp :: ('a,'v) expr => (('a,'v) instr) list
primrec
"comp (Cex v)       = [Const v]"
"comp (Vex a)       = [Load a]"
"comp (Bex f e1 e2) = (comp e2) @ (comp e1) @ [Apply f]"