Add operator overloading for symbolic::Variable

Question

This is for https://github.com/RobotLocomotion/drake/issues/4619. We need operator overloading +, -, *, / between symbolic::Variables. For example,

c++
symbolic::Expression operator+(symbolic::Variable& lhs, symbolic::Variable& rhs)

该提问来源于开源项目：RobotLocomotion/drake

Answer 1

-tri , an easy way to achieve this is to make the following constructor non-explicit:

c++
/** Constructs a variable. */
explicit Expression(const Variable& var);

Will that be fine?