Fix test for interger powers to work with non factor of 2 values.

        bool is_integer() {

            const auto temp = this->evaluate().at(0);

            return std::imag(temp) == 0 &&

                   static_cast<size_t> (std::real(temp))%2 == 0;

                   fmod(std::real(temp), 1.0) == 0.0;

        }

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