public void SolveEquation_CallsParser()
        {
            var equationParserStub = new EquationParserStub();
            var equationSolver     = new EquationSolver(equationParserStub);

            var solution = equationSolver.SolveEquation("blahblahblahblah");

            Assert.IsTrue(equationParserStub.called);
        }