Due to these limitations in each system, where both lexical and expressive categories can only be one layer deep, the recursive and unbounded hierarchical structure of human language is surprising. The Integration hypothesis posits that it is the combination of these two types of layers that results in such a rich hierarchical structure. The alternation between L layers and E layers is what allows human language to reach an arbitrary depth of layers. For example, in the phrase "Eat the cake that Mary baked", the tree structure shows an alternation between L and E layers. This can easily be described by two phrase rules: (i) LP → L EP and (ii) EP → E LP. The recursion that is possible is plainly seen by transforming these phrase rules into bracket notation. The LP in (i) can be written as EP. Then, adding an E layer to this LP to create an EP would result in [L EP]. After, a more complex LP could be obtained by adding an L layer to the EP, resulting in [E [L EP]]. This can continue forever and would result in the recognizable deep structures found in human language.