- Fix BisonSyntaxError location tracking.

 - Sort polynom by its exponents?

 - MESSAGES needs to be expanded.

 - Fix division by zero caused by "0/0": Catch exception in front-end

smvv@multivac ~/work/trs $ printf "a/0\n??" | ./main.py
Traceback (most recent call last):
  File "./main.py", line 75, in <module>
    main()
  File "./main.py", line 64, in main
    node = p.run(debug=args.debug)
  File "/home/smvv/work/trs/external/pybison/src/python/bison.py", line 258, in run
    self.report_last_error(filename, e)
  File "/home/smvv/work/trs/external/pybison/src/python/bison.py", line 251, in run
    self.engine.runEngine(debug)
  File "bison_.pyx", line 592, in bison_.ParserEngine.runEngine (build/external/pybison/bison_.c:592)
  File "/home/smvv/work/trs/src/parser.py", line 195, in hook_handler
    possibilities = handler(retval)
  File "/home/smvv/work/trs/src/rules/fractions.py", line 23, in match_constant_division
    raise ZeroDivisionError('Division by zero: %s.' % node)
ZeroDivisionError: Division by zero: a / 0.

smvv@multivac ~/work/trs $ printf "0/0\n??" | ./main.py
Traceback (most recent call last):
  File "./main.py", line 75, in <module>
    main()
  File "./main.py", line 64, in main
    node = p.run(debug=args.debug)
  File "/home/smvv/work/trs/external/pybison/src/python/bison.py", line 258, in run
    self.report_last_error(filename, e)
  File "/home/smvv/work/trs/external/pybison/src/python/bison.py", line 251, in run
    self.engine.runEngine(debug)
  File "bison_.pyx", line 592, in bison_.ParserEngine.runEngine (build/external/pybison/bison_.c:592)
  File "/home/smvv/work/trs/src/parser.py", line 195, in hook_handler
    possibilities = handler(retval)
  File "/home/smvv/work/trs/src/rules/numerics.py", line 73, in match_divide_numerics
    divide = not divmod(n.value, dv)[1]
ZeroDivisionError: integer division or modulo by zero

 - Last possibilities reduce to a similar result.

smvv@multivac ~/work/trs $ printf "0/1\n??" | ./main.py
<Possibility root="0 / 1" handler=divide_numerics args=(0, 1)>
Division of 0 by 1 reduces to 0.
Division of 0 by 1 reduces to 0.

 - Fractions constant rewrite rules.

 - >>> (sin x) ^ 2 + (cos x) ^ 2
   sin(x) ^ 2 + cos(x) ^ 2
   >>> sin(x) ^ 2 + cos(x) ^ 2
   sin(x ^ 2) + cos(x ^ 2)