sentences:  This means that both "PRO" and ""pro"" are held to be entries in the mental lexicon, whereas DP-trace and Wh-trace, and the null heads are not categories in the lexicon.The empty category subclass called PRO, referred to as "big pro" when speaking, is a DP which appears in a caseless position.PRO is a universal lexical element, it is said to be able to occur in every language, in an environment with a non-finite embedded clause.