use a thread to increase the stack size

(yes, i know this is a hack.  but we can solve the hole6 problem now,
without having to think further. :)

rust/solve/examples/external-03-hole6.cnf

@@ -0,0 +1,148 @@
c http://people.sc.fsu.edu/~jburkardt/data/cnf/hole6.cnf
c File:  hole6.cnf
c
c SOURCE: John Hooker (jh38+@andrew.cmu.edu)
c
c DESCRIPTION: Pigeon hole problem of placing n (for file holen) pigeons
c              in n+1 holes without placing 2 pigeons in the same hole
c
c NOTE: Part of the collection at the Forschungsinstitut fuer 
c       anwendungsorientierte Wissensverarbeitung in Ulm Germany.
c
c NOTE: Not satisfiable
c
p cnf 42 133
-1     -7    0
-1     -13   0
-1     -19   0
-1     -25   0
-1     -31   0
-1     -37   0
-7     -13   0
-7     -19   0
-7     -25   0
-7     -31   0
-7     -37   0
-13    -19   0
-13    -25   0
-13    -31   0
-13    -37   0
-19    -25   0
-19    -31   0
-19    -37   0
-25    -31   0
-25    -37   0
-31    -37   0
-2     -8    0
-2     -14   0
-2     -20   0
-2     -26   0
-2     -32   0
-2     -38   0
-8     -14   0
-8     -20   0
-8     -26   0
-8     -32   0
-8     -38   0
-14    -20   0
-14    -26   0
-14    -32   0
-14    -38   0
-20    -26   0
-20    -32   0
-20    -38   0
-26    -32   0
-26    -38   0
-32    -38   0
-3     -9    0
-3     -15   0
-3     -21   0
-3     -27   0
-3     -33   0
-3     -39   0
-9     -15   0
-9     -21   0
-9     -27   0
-9     -33   0
-9     -39   0
-15    -21   0
-15    -27   0
-15    -33   0
-15    -39   0
-21    -27   0
-21    -33   0
-21    -39   0
-27    -33   0
-27    -39   0
-33    -39   0
-4     -10   0
-4     -16   0
-4     -22   0
-4     -28   0
-4     -34   0
-4     -40   0
-10    -16   0
-10    -22   0
-10    -28   0
-10    -34   0
-10    -40   0
-16    -22   0
-16    -28   0
-16    -34   0
-16    -40   0
-22    -28   0
-22    -34   0
-22    -40   0
-28    -34   0
-28    -40   0
-34    -40   0
-5     -11   0
-5     -17   0
-5     -23   0
-5     -29   0
-5     -35   0
-5     -41   0
-11    -17   0
-11    -23   0
-11    -29   0
-11    -35   0
-11    -41   0
-17    -23   0
-17    -29   0
-17    -35   0
-17    -41   0
-23    -29   0
-23    -35   0
-23    -41   0
-29    -35   0
-29    -41   0
-35    -41   0
-6     -12   0
-6     -18   0
-6     -24   0
-6     -30   0
-6     -36   0
-6     -42   0
-12    -18   0
-12    -24   0
-12    -30   0
-12    -36   0
-12    -42   0
-18    -24   0
-18    -30   0
-18    -36   0
-18    -42   0
-24    -30   0
-24    -36   0
-24    -42   0
-30    -36   0
-30    -42   0
-36    -42   0
 6      5      4      3      2      1    0
 12     11     10     9      8      7    0
 18     17     16     15     14     13   0
 24     23     22     21     20     19   0
 30     29     28     27     26     25   0
 36     35     34     33     32     31   0
 42     41     40     39     38     37   0

rust/solve/src/main.rs

@@ -12,6 +12,7 @@ extern crate solve;
use std::io;
use std::io::Read;
use std::thread;
use solve::cnf;
use solve::dpll;
@@ -26,10 +27,12 @@ fn main() {
		    println!("{:?}", clause);
		}
                match dpll::dpll(cnf.clauses) {
                    Some(bindings) => println!("satisfiable: {:?}", bindings),
                    None => println!("not satisfiable")
                }
                thread::Builder::new().name("solver".to_string()).stack_size(100 * 1024 * 1024).spawn(move || {
                    match dpll::dpll(cnf.clauses) {
                        Some(bindings) => println!("satisfiable: {:?}", bindings),
                        None => println!("not satisfiable")
                    }
                }).unwrap().join();
	    }
	    Err(e) => { println!("Error: {}", e) }
	},