doc-src/Tutorial/Recdef/fib

fully enclose "\isadigit{...}";
\<^foo> produces "\isactrlfoo ";

consts fib :: nat => nat
recdef fib "measure(%n. n)"
  "fib 0 = 0"
  "fib 1 = 1"
  "fib (Suc(Suc x)) = fib x + fib (Suc x)"