z.mant[n-1] |= msb
			}
		}
	}

	// zero out trailing bits in least-significant word
	z.mant[0] &^= lsb - 1

	if debugFloat {
		z.validate()
	}
}

func (z *Float) setBits64(neg bool, x uint64) *Float {
	if z.prec == 0 {
		z.prec = 64
	}
	z.acc = Exact
	z.neg = neg
	if x == 0 {
		z.form = zero
		return z
	}
	// x != 0
	z.form = finite