void mux41(int s1, int s2,                  // selection inputs
           int i1, int i2, int i3, int i4,  // inputs
           int *out)                        // output
{
    switch((s2 << 1) | s1)
    {
        case 0:
            *out = i1;
            break;
        
        case 1:
            *out = i2;
            break;
        
        case 2:
            *out = i3;
            break;
        
        case 3:
            *out = i4;
            break;
    }
}