--- a/once.pl	Wed Dec 16 17:25:38 2015 +0100
+++ b/once.pl	Sat Dec 19 23:36:51 2015 +0100
@@ -302,7 +302,8 @@
 }
 
 sub base62 {
-    my $n = shift;
+    my $n = shift // $_;
+    die 'left integer precision' if $n == $n - 1 or $n == $n + 1;
     state $digits = [0..9, 'a'..'z', 'A'..'Z'];
     state $base = @$digits;
     my @result;