title: Simple Maths Expression Parser

## Aim
Parse each of the following lines
```
1 + 2
(1 + 2) * 3
4 * (5 - 6)
7 + 8 + 9
5 / (3 + 1)
```
## Notes
Antlr4 has the mandatory convention that [lexer](../../lexers) patterns begin with a Capital Letter and syntactic
patterns start with a lowercase letter.

## Example Grammar
This is my first attempt.
```antlr
grammar Expr;

DIGIT: [0-9] ;
NUMBER: DIGIT+ ;

expr: expr ('+'|'-') term | term ;
term: term ('*'|'/') NUMBER | NUMBER | '(' expr ')';
```
Problems:
1. It doesn't handle whitespace (probably want `WHITESPACE: [ \t\r\n] -> skip;`).