= Identifier

In <<StandardML#,Standard ML>>, there are syntactically two kinds of
identifiers.

* Alphanumeric: starts with a letter or prime (`'`) and is followed by letters, digits, primes and underbars (`_`).
+
Examples: `abc`, `ABC123`, `Abc_123`, `'a`.

* Symbolic: a sequence of the following
+
----
 ! % & $ # + - / : < = > ? @ | ~ ` ^ | *
----
+
Examples: `+=`, `\<=`, `>>`, `$`.

With the exception of `=`, reserved words can not be identifiers.

There are a number of different classes of identifiers, some of which
have additional syntactic rules.

* Identifiers not starting with a prime.
** value identifier (includes variables and constructors)
** type constructor
** structure identifier
** signature identifier
** functor identifier
* Identifiers starting with a prime.
** type variable
* Identifiers not starting with a prime and numeric labels (`1`, `2`, ...).
** record label
