# BACKLOG — phase `lex`

## Build backlog

All items completed.

- [x] D1: Implement `NUMBER` token (int + float, including `.5` and `10.`)
- [x] D2: Implement operator and paren tokens (`PLUS`, `MINUS`, `STAR`, `SLASH`, `LPAREN`, `RPAREN`)
- [x] D3: Skip whitespace; raise `LexError` for invalid characters
- [x] D4: Write `calc/test_lexer.py` with unittest coverage for D1–D3