# Lexical Structure

## Keywords

All Arend’s keywords begin with \. Here’s the complete list of keywords:

## Numerals

A positive numeral is a non-empty sequence of digits. A negative numeral consists of symbol - followed by a non-empty sequence of digits.

## Identifiers

An identifier consists of a non-empty sequence of lower and upper case letters, digits, and characters from the list ~!@#$%^&*-+=<>?/|[]:_. Exceptions are sequences that begin with a digit or symbol ', numerals, and reserved names such as ->, =>, _, :, and |. Examples: • Valid identifiers: xxx, +, $^~]!005x, ::, ->x, x:Nat, -5b, -33+7, --xxx.
• Invalid identifiers: 5b, -33, =>.

## Infix and postfix notation

A postfix notation is an identifier which follows . An infix notation is an identifier surrounded by . Both of these notations are described in Definitions.

Multi-line comments are enclosed in {- and -} and can be nested. Single-line comments consist of a sequence of symbols - of length at least 2 followed by a whitespace followed by an arbitrary text until the end of the line. To give an example, --, --------, -- foo, and ------- foobar are comments but --foo, foo--bar, and ------foobar are not.