Erand has `thr` parameter that's unused & undocumented

[siboehm]

// Float64 returns, as a float64, a pseudo-random number in the half-open interval [0.0,1.0).
func (r *SysRand) Float64(thr int) float64 {
	if r.Rand == nil {
		return rand.Float64()
	}
	return r.Rand.Float64()
}

[rcoreilly]

This is documented in the Rand interface:

// All methods take an optional thr argument for the
// thread number in the parallel threaded case.

this is unused for the std go rng but is used in e.g., slrand -- allows for interface to support that kind in the future.