slice_lhs a b { tac } zooms to the left hand side, uses associativity for categorical
composition as needed, zooms in on the a-th through b-th morphisms, and invokes tac.
slice_rhs a b { tac } zooms to the right hand side, uses associativity for categorical
composition as needed, zooms in on the a-th through b-th morphisms, and invokes tac.