If letters are used for predicate constants as well as for predicate variables, then there has to be a way of distinguishing between them.
2.
Predicate variables should be distinguished from predicate constants, which could be represented either with a different ( exclusive ) set of predicate letters, or by their own symbols which really do have their own specific meaning in their domain of discourse : e . g . =, \ \ in, \ \ le, \.
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