For a TSL 2 grammar G = T, S , T ⊆ Σ will be referred to as the tier, S the allowed tier substrings, and R = fac T-2 (Σ * ) − S as the forbidden tier substrings.
these are words which may contain bs interspersed with as and cs provided that no a precedes another without an intervening c. For example, bbabbcbba ∈ L but bbabbbbabb ∈ L, because E t (bbabbbbabb) = aa and aa ∈ fac 2 ( aa ) but aa ∈ S. Like the class of SL k languages, the class of tSL languages (for fixed t and k) is a string extension language class.