return the list of constraints which are of complementary kinds of a given AMPL problem
cvar = ampl_get_compl(asl)
a structure which handles the pointer to the problem
the list of index of the complementarity constraints. cvar[i] > 0 means constraint i complements variable cvar[i] - 1. If cvar==-1 then, there are no complementarity constraints.
Return the list of constraints which are of complementary kinds of a given AMPL problem.