My bad, it's a file from Sightseer. Basically a copy of Mathf, but with double precision.
You can resolve it by replacing MathD with adding a Clamp function:
static public double Angle (Vector3D from, Vector3D to)
{
return Math.Acos(Clamp(Dot(from.normalized, to.normalized), -1d, 1d)) * 57.29578d;
}
static double Clamp (double value, double min, double max)
{
if (value < min) value = min;
else if (value > max) value = max;
return value;
}
The other MathD usage can be replaced with Math.