Negate this.
Source
void negate() {
_m4storage[0] = -_m4storage[0];
_m4storage[1] = -_m4storage[1];
_m4storage[2] = -_m4storage[2];
_m4storage[3] = -_m4storage[3];
_m4storage[4] = -_m4storage[4];
_m4storage[5] = -_m4storage[5];
_m4storage[6] = -_m4storage[6];
_m4storage[7] = -_m4storage[7];
_m4storage[8] = -_m4storage[8];
_m4storage[9] = -_m4storage[9];
_m4storage[10] = -_m4storage[10];
_m4storage[11] = -_m4storage[11];
_m4storage[12] = -_m4storage[12];
_m4storage[13] = -_m4storage[13];
_m4storage[14] = -_m4storage[14];
_m4storage[15] = -_m4storage[15];
}