```
module main {
    var x : int;

    init {
        x = 3;
    }

    type action_t = enum {IDLE, ACTION_A, ACTION_B};
    var action : action_t;

    assign {
        // Split based on actions
        case 
        action == ACTION_A: x' = 2 * x;
        action == ACTION_B: x' = x + 1;
        action == IDLE: x' = x;
        esac;
    }

    next(action) {
        action' = nondet(action_t);
    }

    control {
        action = IDLE;
        while (*) {
            if (*) {
                action = ACTION_A;
            } else {
                action = ACTION_B;
            }
        }
    }
}
```
In this model, `x` is the shared variable initialized to 3. The system can perform two simultaneous actions (ACTION_A and ACTION_B) which modify `x`. ACTION_A doubles the value of `x`, and ACTION_B increments `x` by 1. There is also an idle action (IDLE) which keeps `x` unchanged. In each iteration of the loop, either ACTION_A or ACTION_B is non-deterministically chosen for execution, simulating the concurrent behavior in the original program description using the nondetermism of UCLID5's `action' = nondet(action_t);`. This code segment provides a basic modelling for the concurrent operations on `x`.