constant @Stern-Brocot = 1, 1, {
    |(@_[$_ - 1] + @_[$_], @_[$_]) given ++$
} ... *;
 
say @Stern-Brocot[^15];
 
for (flat 1..10, 100) -> $ix {
    say "first occurrence of $ix is at index : ", 1 + @Stern-Brocot.first($ix, :k);
}
 
say so 1 == all map ^1000: { [gcd] @Stern-Brocot[$_, $_ + 1] }