improve doctest coverage of logic/logic.py

[mvngu]

As the subject says. Currently, the coverage of sage/logic/logic.py in Sage 4.4 is:

[mvngu@sage sage-4.4]$ ./sage -coverage devel/sage-main/sage/logic/logic.py 
----------------------------------------------------------------------
devel/sage-main/sage/logic/logic.py
ERROR: Please add a `TestSuite(s).run()` doctest.
SCORE devel/sage-main/sage/logic/logic.py: 16% (3 of 18)

Missing documentation:
 * combine(self, statement1, statement2):   see #15262
 * simplify(self, table):
 * prove(self, statement):


Missing doctests:
 * get_bit(x, c):
 * eval(toks):
 * eval_ltor_toks(lrtoks):
 * reduce_bins(lrtoks):
 * reduce_monos(lrtoks):
 * eval_mon_op(args):
 * eval_bin_op(args):
 * eval_and_op(lval, rval):
 * eval_or_op(lval, rval):
 * eval_ifthen_op(lval, rval):
 * eval_iff_op(lval, rval):
 * tokenize(s, toks):

This needs to be coordinated with #8797.

[riccardomurri]

Add docstring to functions combine, simplify and prove.

[riccardomurri]

Functions simplify, combine and prove are actually not implemented: the code is a stub. Patch sage-issue-8790-not-implemented.diff provides a docstring stating that.

I'm not sure how doctests should be provided here: one could probably change the code to raise a NotImplemented exception and look for that in the doctest.

[riccardomurri]

Rename internal functions to start with an underscore

[riccardomurri]

The get_bits, eval_*, reduce_* and tokenize functions are explictly described as "for internal use only". It is inconvenient to provide a doctest for them, as arguments are the internal data structures used in the SymbolicLogic class.

Patch sage-issue-8790-module-private-funcs.diff​ renames the functions to begin with an underscore to make their non-public nature more evident, in the hope to silence the coverage warning.

[LostPw]

fix for the combine function #15262

[aapitzsch]

I get

./sage --coverage src/sage/logic/logic.py 
------------------------------------------------------------------------
SCORE src/sage/logic/logic.py: 100.0% (18 of 18)
------------------------------------------------------------------------