[Cbc] SOS constraints

[Miles Lubin]

Hi all,

I just tried to clean up and test SOS constraints in the new C
interface. My test problem is:

Minimize  5x[1] + 3x[2] + 2x[3] + 7x[4] + 4x[5]
     s.t.       x[1] +  x[2] +  x[3] +  x[4] +  x[5] == 1
               All x binary

I have not added the equality constraint explicitly to the model, but
instead added an SOS 1 constraint. When I solve, the solution is zero,
so the SOS 1 constraint doesn't seem to be enforced. Is adding the SOS
object to the model sufficient to enforce the constraint, or is it
purely for branching?

Test code: https://projects.coin-or.org/Cbc/browser/trunk/Cbc/test/CInterfaceTest.c?rev=2033#L113
Implementation of Cbc_addSOS in the C interface:
https://projects.coin-or.org/Cbc/browser/trunk/Cbc/src/Cbc_C_Interface.cpp#L827

Thanks,
Miles