Identifiers are the names of functions, variables and data structures.
Their format must, of course, follow the specifications of the rest of the text.
Identifiers contains letters, digits and underscores. An identifier must start with a letter.
If an identifier is a reserved word in a language, it is appended with
x when writing to the language. If an identifier is a reserved word and
x is appended, etc. When reading from a language, the reverse is done.