Fix number of variables in DIMACS
Add test script

user@hp:~/xxx/sym-to-cnf$ ./test.sh
  20,603,435,688 bytes allocated in the heap
   1,499,913,704 bytes copied during GC
     201,869,456 bytes maximum residency (12 sample(s))
       5,187,120 bytes maximum slop
             557 MB total memory in use (0 MB lost due to fragmentation)

                                    Tot time (elapsed)  Avg pause  Max pause
  Gen  0     39848 colls,     0 par    0.94s    1.29s     0.0000s    0.0010s
  Gen  1        12 colls,     0 par    0.90s    1.12s     0.0933s    0.3592s

  INIT    time    0.00s  (  0.04s elapsed)
  MUT     time    7.59s  (  7.30s elapsed)
  GC      time    1.83s  (  2.41s elapsed)
  EXIT    time    0.00s  (  0.05s elapsed)
  Total   time    9.42s  (  9.80s elapsed)

  %GC     time      19.4%  (24.6% elapsed)

  Alloc rate    2,714,530,486 bytes per MUT second

  Productivity  80.6% of total user, 77.5% of total elapsed

  56,416,722,144 bytes allocated in the heap
   3,716,792,400 bytes copied during GC
     552,348,704 bytes maximum residency (13 sample(s))
      11,669,568 bytes maximum slop
            1557 MB total memory in use (0 MB lost due to fragmentation)

                                    Tot time (elapsed)  Avg pause  Max pause
  Gen  0     109185 colls,     0 par    2.63s    3.49s     0.0000s    0.0016s
  Gen  1        13 colls,     0 par    2.19s    2.75s     0.2117s    0.8763s

  INIT    time    0.00s  (  0.00s elapsed)
  MUT     time   21.54s  ( 20.81s elapsed)
  GC      time    4.82s  (  6.25s elapsed)
  EXIT    time    0.00s  (  0.14s elapsed)
  Total   time   26.36s  ( 27.20s elapsed)

  %GC     time      18.3%  (23.0% elapsed)

  Alloc rate    2,619,093,247 bytes per MUT second

  Productivity  81.7% of total user, 79.2% of total elapsed
Slightly improve memory efficiency

user@hp:~/xxx/sym-to-cnf$ ./test.sh
  11,238,389,024 bytes allocated in the heap
   1,024,739,672 bytes copied during GC
     115,754,920 bytes maximum residency (12 sample(s))
       3,544,568 bytes maximum slop
             332 MB total memory in use (0 MB lost due to fragmentation)

                                    Tot time (elapsed)  Avg pause  Max pause
  Gen  0     21752 colls,     0 par    0.50s    0.83s     0.0000s    0.0011s
  Gen  1        12 colls,     0 par    0.49s    0.58s     0.0487s    0.1752s

  INIT    time    0.00s  (  0.00s elapsed)
  MUT     time    4.73s  (  4.46s elapsed)
  GC      time    0.99s  (  1.41s elapsed)
  EXIT    time    0.00s  (  0.03s elapsed)
  Total   time    5.72s  (  5.91s elapsed)

  %GC     time      17.3%  (23.9% elapsed)

  Alloc rate    2,376,875,235 bytes per MUT second

  Productivity  82.7% of total user, 80.1% of total elapsed

  31,343,173,984 bytes allocated in the heap
   2,678,093,800 bytes copied during GC
     399,340,008 bytes maximum residency (13 sample(s))
       9,652,776 bytes maximum slop
            1011 MB total memory in use (0 MB lost due to fragmentation)

                                    Tot time (elapsed)  Avg pause  Max pause
  Gen  0     60753 colls,     0 par    1.48s    2.36s     0.0000s    0.0010s
  Gen  1        13 colls,     0 par    1.22s    1.55s     0.1189s    0.4967s

  INIT    time    0.00s  (  0.00s elapsed)
  MUT     time   13.82s  ( 13.12s elapsed)
  GC      time    2.70s  (  3.91s elapsed)
  EXIT    time    0.00s  (  0.08s elapsed)
  Total   time   16.52s  ( 17.11s elapsed)

  %GC     time      16.3%  (22.8% elapsed)

  Alloc rate    2,267,651,578 bytes per MUT second

  Productivity  83.7% of total user, 80.8% of total elapsed