An identifier is a sequence of one or more alphanumeric characters and underscores that is used to name a language construct. The exceptions are:
A single underscore character does not form an identifier. Using a single underscore instead of an identifier means no identifier is given to this construct.
Cairo keywords cannot be used as identifiers.
Identifiers starting with an underscore are typically used to indicate an identifier that is intentionally unused, and will silence the unused warning.