docs: function(a, b, stop) {
           var A, A1, A2, // A2 is "two previous" to A, A1 is "one previous"
               B, B1, B2, an, bn, i;
  
           an = a(0);
           A = an;
           A1 = 1;
           bn = 1;
           B = bn;
           B1 = 0;
           i = 0;
           return utils.repeat(A / B, function() {
              A2 = A1;
              A1 = A;
              B2 = B1;
              B1 = B;
              i += 1;
              an = a(i, an);
              bn = b(i, bn);
              A = A1 * an + A2 * bn;
              B = B1 * an + B2 * bn;
              return A / B;
           }, stop);
        }