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int user_select_child(void *user, double ub, branch_obj *can, char *action)
By the time this function is invoked, the candidate for branching has been chosen. Based on this information and the current best upper bound, the user has to decide what to do with each child. Possible actions for a child are KEEP_THIS_CHILD (the child will be kept at this LP for further processing, i.e., the process dives into that child), PRUNE_THIS_CHILD (the child will be pruned based on some problem specific property--no questions asked...), PRUNE_THIS_CHILD_FATHOMABLE (the child will be pruned based on its pre-solved LP relaxation) and RETURN_THIS_CHILD (the child will be sent back to tree manager). Note that at most one child can be kept at the current LP process.
There are two default options--in both of them, objective values of the pre-solved LP relaxations are compared (for those children whose pre-solve did not terminate with primal infeasibility or high cost). One rule prefers the child with the lowest objective function value and the other prefers the child with the higher objective function value.
Checks which children can be fathomed based on the objective value of their pre-solved LP relaxation.