void add_spring (Real, Real);
Real range_ideal_len (int l, int r) const;
Real range_stiffness (int l, int r) const;
void add_spring (Real, Real);
Real range_ideal_len (int l, int r) const;
Real range_stiffness (int l, int r) const;
/* returns a vector of dimensions breaks.size () * breaks.size () */
vector<Real> get_line_forces (vector<Grob*> const &columns,
/* returns a vector of dimensions breaks.size () * breaks.size () */
vector<Real> get_line_forces (vector<Grob*> const &columns,