(defun num_trees (num)
  (if (num >= 3)
    (* (- (* 2 num) 5) (num_trees (- num 1)))))

sub Num_Trees {
  my $taxa = shift;
  my $trees = 1;
  while ($taxa >= 3) {
    $trees = (($taxa * 2) - 5) * $trees;
    $taxa--;
  }
  return($trees);
}

long num_trees( int argc , char **argv )  {
  int taxa = atoi(argv[1]);
  long trees = 1.0;
  int t;
  for ( t = 3 ; t <= taxa ; ++t ) {
    trees = (trees * ((taxa * 2) - 5));
  }
  return(trees);
}

fun num_trees( 3 : int ) = 1 : int
        | num_trees( num ) = if num < 3 then 0;
        | num_trees( num ) = if num > 3 then ((2 * n) - 5) * num_trees(num - 1);