4 _a(a),_b(b),_above(above),_under(under){}
 double operator()(const Vector2D &r) const
Evaluates the function. 
Line2D(double a, double b, double above, double under)
Class constructor. 
double y
Component in the y direction. 
A spatial distribution with a Line step function. 
double x
Component in the x direction.