Directly applies the operator op
to the state a
by contracting the physical indices and merging the auxiliary ones, without truncation.
References syten::BTT::State::aIdx, apply_operator_directly_rec(), syten::BTT::State::bIdx, syten::BTT::State::drop_unmatched(), syten::genCombinator(), syten::BTT::State::maybeCache(), syten::BTT::State::pIdx, syten::BTT::State::pleaseCache, syten::BinaryTree::Tree< T >::r, syten::BTT::State::rIdx, and syten::BinaryTree::Node< T >::v.
Referenced by apply_op_naive(), syten::BTT::State::load(), and operator*=().