ADMUX = ADMUX | _BV(REFS); // uncompounded bitwise OR
ADMUX |= _BV(REFS0); // #define _BV(bit) (1 << (bit))
ADMUX |= bit(REFS0); // #define bit(b) (1UL << (b))
bitSet(ADMUX, REFS0); // #define bitSet(value, bit) ((value) |= (1UL << (bit)))
sbi(ADMUX, REFS0); // deprecated assembly?
ADMUX |= (1 << REFS0); // shift operator
ADMUX |= (1 << 7); // bit 7 is named REFS0
ADMUX |= 0b10000000; // bin
ADMUX |= 0x80; // hex
ADMUX |= 128; // dec
*under normal 8 bit Arduino circumstances?