public bool IsVertex(Int3 p, out int index)
{
index = -1;
if (v0.IsEqualXZ(ref p))
{
index = 0;
}
else if (v1.IsEqualXZ(ref p))
{
index = 1;
}
else if (v2.IsEqualXZ(ref p))
{
index = 2;
}
return(index != -1);
}