///\todo This might by implemented to be usable in other places.

   ///\todo This might be implemented to be also usable in other places.

     ///Linear expression

     ///Linear expression of variables and a constant component

     //    typedef SparseLinExpr<Col> Expr;

     ///This data structure strores a linear expression of the variables

     ///(\ref Col "Col"s) and also has a constant component.

     ///

     ///There are several ways to access and modify the contents of this

     ///container.

     ///- Its it fully compatible with \c std::map<Col,double>, so for expamle

     ///if \c e is an Expr and \c v and \c w are of type \ref Col then you can

     ///read and modify the coefficients like

     ///these.

     ///\code

     ///e[v]=5;

     ///e[v]+=12;

     ///e.erase(v);

     ///\endcode

     ///or you can also iterate through its elements.

     ///\code

     ///double s=0;

     ///for(LpSolverBase::Expr::iterator i=e.begin();i!=e.end();++i)

     ///  s+=i->second;

     ///\endcode

     ///(This code computes the sum of all coefficients).

     ///- Numbers (<tt>double</tt>'s)

     ///and variables (\ref Col "Col"s) directly convert to an

     ///\ref Expr and the usual linear operations are defined so  

     ///\code

     ///v+w

     ///2*v-3.12*(v-w/2)+2

     ///v*2.1+(3*v+(v*12+w+6)*3)/2

     ///\endcode

     ///are valid expressions. The usual assignment operations are also defined.

     ///\code

     ///e=v+w;

     ///e+=2*v-3.12*(v-w/2)+2;

     ///e*=3.4;

     ///e/=5;

     ///\endcode

     ///- The constant member can be set and read by \ref constComp()

     ///\code

     ///e.constComp()=12;

     ///double c=e.constComp();

     ///\endcode

     ///

     ///\note that \ref clear() not only sets all coefficients to 0 but also

     ///clears the constant components.