code: func (curve *KoblitzCurve) IsOnCurve(x, y *big.Int) bool {
  	// Convert big ints to field values for faster arithmetic.
  	fx, fy := curve.bigAffineToField(x, y)
  
  	// Elliptic curve equation for secp256k1 is: y^2 = x^3 + 7
  	y2 := new(fieldVal).SquareVal(fy).Normalize()
  	result := new(fieldVal).SquareVal(fx).Mul(fx).AddInt(7).Normalize()
  	return y2.Equals(result)
  }
docs: // IsOnCurve returns boolean if the point (x,y) is on the curve.
  // Part of the elliptic.Curve interface. This function differs from the
  // crypto/elliptic algorithm since a = 0 not -3.