sentences:  The base case is the predicates of "X" applied to some variables.One always implicitly has a predicate formula_16 for each letter of an alphabet, stating that the letter at position is an .The semantics of the formulae in FO is straightforward, formula_17 is true iff is false, formula_18 is true iff is true and is true, and formula_19 is true iff formula_20 is true for all values that may take in the underlying universe.