// Check if P is treated like zero (if possible).
	// y^2 = x^3 - 3x + B
	// y = mod_sqrt(x^3 - 3x + B)
	// y = mod_sqrt(B) if x = 0
	// If there is no modsqrt, there is no point with x = 0, can't test x = P.
	if yy := new(big.Int).ModSqrt(curve.Params().B, p); yy != nil {
		if !curve.IsOnCurve(big.NewInt(0), yy) {
			t.Fatal("(0, mod_sqrt(B)) is not on the curve?")
		}
		checkIsOnCurveFalse("P, y", p, yy)
	}
}

		b.Mul(&b, &g).Mod(&b, p)
		r = m
	}
}

// ModSqrt sets z to a square root of x mod p if such a square root exists, and
// returns z. The modulus p must be an odd prime. If x is not a square mod p,
// ModSqrt leaves z unchanged and returns nil. This function panics if p is
// not an odd integer, its behavior is undefined if p is odd but not prime.
func (z *Int) ModSqrt(x, p *Int) *Int {
	switch Jacobi(x, p) {
	case -1:

	for m := range isExplicit {
		vendorMods = append(vendorMods, m)
	}
	for m := range modpkgs {
		if !isExplicit[m] {
			vendorMods = append(vendorMods, m)
		}
	}
	gover.ModSort(vendorMods)

	var (
		buf bytes.Buffer
		w   io.Writer = &buf
	)
	if cfg.BuildV {
		w = io.MultiWriter(&buf, os.Stderr)
	}

	if modload.MainModules.WorkFile() != nil {

	for rootsDirty && len(conflicts) == 0 {
		roots = roots[:0]
		for p, v := range selectedRoot {
			if v != "none" {
				roots = append(roots, module.Version{Path: p, Version: v})
			}
		}
		gover.ModSort(roots)

		// First, we extend the graph so that it includes the selected version
		// of every root. The upgraded roots are in addition to the original
		// roots, so we will have enough information to trace a path to each

pkg math/big, method (*Float) Sub(*Float, *Float) *Float
pkg math/big, method (*Float) Text(uint8, int) string
pkg math/big, method (*Float) Uint64() (uint64, Accuracy)
pkg math/big, method (*Int) ModSqrt(*Int, *Int) *Int
pkg math/big, method (Accuracy) String() string
pkg math/big, method (ErrNaN) Error() string
pkg math/big, method (RoundingMode) String() string
pkg math/big, type Accuracy int8