Product of two operators, \( \hat O \; \hat T \), *this
will be applied first to the state.
References syten::compat_exact_equal(), dirs, dropUnmatched(), is_sso, isIdentity(), isSingleSite(), syten::MPS::mpo_cmp_prod(), singleSite(), std::vector< Tensor< 4 > >::size(), and SYTEN_ASSERT.