We define top(τ) = X and yield(τ) = w L w R , where juxtaposition denotes string concatenation.
at this point, only some rule of type (26) can be applied, resulting in the reduction of the stack, immediately followed by some rule of type (25), which is a composition rule of degree m. If the derivation were to use more than one occurrence of (26), then it would derive a category whose stack contains fewer than m elements.