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  DB<288> sub fac {my $x=1; $x*=$_ for 2..$_[0]; $x}

...

  DB<290> binom 26,8
 => 1562275

a bcd
 abcd
aa cd
aab d
aabc

  DB<292> length "aaaabbbcccddddeeeeffffgggghhhiiijjkkkllllmmmnnnnoooo
+pppqrrrrssssstttuuuuvvwwwxxyyyzzzz"
 => 86

  DB<293> binom 86,8
 => "53060358690"

 "aaaabbbc..." # pattern
 "1000100...." # ok      => ab...
 "1100100...." # ok      => aab...
 "1001100....  # not ok  => aab...

use strict;
use warnings;
...
generate(0,"");

print $nsolutions;