On the model-theoretic side, an innovative feature of DL is the move to a multi-sorted interpretation.  The key notion is that of a graded algebra: a freely generated algebra (W,·,1, ⎵) where the monoid (W,·,1) of the interpretation of L* is augmented with a distinguished generator ⎵ called the separator. The sort of an expression s, σ(s), is given by the number of occurrences of the separator in it. Expressions of the nullary sort are the familiar strings for the language models of L. Expressions of sort n &gt; 0 are split strings, with n marked positions where other expressions can be substituted.