code: def quick_solve(expr, var):
          r
          res = -(expr - var)
  
          # This may not work every time,
          # so we fallback on the --slow-- `solve()` if we failed
          if var in res.args:
              return sympy.solve(expr, var)
          return res
docs: r"""
          A function that tries to solve a very simple equation in the quickest way.
          For instance, an expression like :math: `2*a + 3*b + c == 0` needed to be solved for :math: `c`
          may be simply solved by stating :math: `c = -(2*a + 3*b)`.
          The function checks if the right hand side does not contain :math: `c`.
          If it does, then, the classic `sympy.solve()` method is used.
          `sympy.solve()` uses simplify in the background (i.e. is slow),
          it is therefore preferable to avoid using it.
  
          :param expr: an expression like implicitly equal to 0
          :param var: a variable to solve for
          :return: the solution for `var`