doc-src/Tutorial/Ifexpr/value

Added an exception handler and error msg.

consts value :: boolex => (nat => bool) => bool
primrec
"value (Const b) env = b"
"value (Var x)   env = env x"
"value (Neg b)   env = (~ value b env)"
"value (And b c) env = (value b env & value c env)"