Sharable Signatures?

convert.p6

#!perl6

my @alphabet = ('0' .. '9', 'A' .. 'Z', 'a' .. 'z').flat;
sub to-digits(Int $n is copy, Int $b where $b >= 2 && $b <= 62 --> Str) {
  my @digits; 
  while $n > 0 {
    @digits.push(@alphabet[$n % $b]);
    $n = $n div $b;
  }

  @digits.reverse.join;
}

sub from-digits(Str $digits, Int $b where $b >= 2 && $b <= 62 --> Int) {
  my $n = 0;
  for $digits.comb -> $d {
    $n = $b * $n + @alphabet.first({ $_ eq $d }, :k);
  }

  $n;
}

sub to-base(Str $n, Int $b where $b >= 2 && $b <= 62, Int $c where $c >= 2 && $c <= 62 --> Str) {
  to-digits(from-digits($n, $b), $c);
}

say to-base('333', 10, 16);
say to-base('14D', 16, 10);
say to-base('333', 10, 62);
say to-base('14D', 16, 62);

I notice that I have the same signature constraint four times on the bases passed to this programs three functions. Is there a way to share that constraint?