Abstraction variable indicator selbrisle.
xe'u is a pro-bridi (meaning it takes the place of a fully-specified bridi). xe'u indicates that the selbrisle is considered to be a bridi-variable that can be filled by other bridi. xe'u makes sense only inside abstraction clauses. In particular, xe'u is almost solely used in prenex in order to put a word of selma'o NU in prenex in a statement of predicate logic of third or higher order. See bu'ai for usage. See also ce'u, bu'ai.