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A.2.3 Identifiers
An identifier is a sequence of letters and digits. The first character must be a letter; the underscore _ counts as a letter. Upper and lower case letters are different. Identifiers may have any length, and for internal identifiers, at least the first 31 characters are significant; some implementations may take more characters significant. Internal identifiers include preprocessor macro names and all other names that do not have external linkage (Par.A.11.2). Identifiers with external linkage are more restricted: implementations may make as few as the first six characters significant, and may ignore case distinctions.