struct fred
{
   int status;
   // ...§
};

#define CONDITION_A    0x01
#define CONDITION_B    0x02
#define CONDITION_C    0x03

#define SET_CONDITION_A(p)    ((p)->status |= CONDITION_A)
#define SET_CONDITION_B(p)    ((p)->status |= CONDITION_B)
#define SET_CONDITION_C(p)    ((p)->status |= CONDITION_C)

#define CLEAR_CONDITION_A(p)  ((p)->status &= ~CONDITION_A)
#define CLEAR_CONDITION_B(p)  ((p)->status &= ~CONDITION_B)
#define CLEAR_CONDITION_C(p)  ((p)->status &= ~CONDITION_C)

#define IN_CONDITION_A(p)     ((p)->status & CONDITION_A)
#define IN_CONDITION_B(p)     ((p)->status & CONDITION_B)
#define IN_CONDITION_C(p)     ((p)->status & CONDITION_C)

#define POSSIBILITIES(x)      ((x) & 0x0030)
#define POSSIBILITY_A          0x0000
#define POSSIBILITY_B          0x0010
#define POSSIBILITY_C          0x0020
#define POSSIBILITY_D          0x0030

enum { possibility_a, possibility_b, possibility_b, possibility_d };

struct fred
{
    unsigned in_condition_a : 1;
    unsigned in_condition_b : 1;
    unsigned in_condition_c : 1;

    unsigned possibilities : 2;
};