process chained equalities with different signs correctly

[dimpase]

we have to deal with what remained from #12091. At least, report errors for the inputs like the following:

sage: p = MixedIntegerLinearProgram()
sage: b = p.new_variable()
sage: b[0] <= 555*b[1] >= 2
sage: b[0] <= 555*b[1] == 2

which even with #12091 are incorrectly processed

sage: b[0] <= 555*b[1] >= 2
2 <= x_0 <= 555*x_1
sage: b[0] <= 555*b[1] == 2
555*x_1 == 2

Depends on #14540

CC: @vbraun @ppurka @nathanncohen

Component: linear programming

Reviewer: Jeroen Demeyer

Issue created by migration from https://trac.sagemath.org/ticket/13696

[mkoeppe]

comment:6

Description modified to make the example self-contained.

[jdemeyer]

Changed dependencies from #12091 to #14540

[mkoeppe]

comment:10

And how about:

sage: b[0] <= 3 <= 4 <= b[1]
4 <= x_1

[jdemeyer]

Reviewer: Jeroen Demeyer

[jdemeyer]

comment:13

The reviewer of #14540 was not happy with a partial solution, so it's probably better to thoroughly fix the problem on #14540.