## [0.23.0] 2020-07-26
### Added
- `p` operator to print the top of the stack via debug function
- `&var` syntax to create variable reference in expression
- `=` operator for variable assignment in expression
- `exists` operator to check for variable existance (only for references)
- `deref` operator to convert variable reference into a value (only for references)
- Allow to use either float or double as floating type, parse numbers in expressions as specified type
- Add boolean type, parse `true` and `false` in expressions
- Add null type, parse `null` in expressions
- Add string type, parse double-quoted `"string"` in expressions
- Add integer and unsigned integer type, used in operators
- Allow to configure underlying types from rpnlib\_config.h and -D... flags
- Return `rpn_error` from operators, split error types into categories
- Create a new stack by using `[` keyword. Move stack contents into the previous stack + size by using `]`.

### Changed
- Stack structure no longer holds raw `float`, but internal `rpn_value` type
- rpn\_... setter and getter methods use `rpn_value` type
- Operator functions return `rpn_error` type, allowing to return both value and operator errors
- Variables in expressions are no longer required to exist when using `&var`
  Expression will automatically create the variable, set it to `null` and push it's reference on the stack
- It is possible to create 'reference' stack values
- Improve precision of `e` and `pi`

### Fixed
- Proper value for `e` constant
- Allow to use multiple contexts simultaniously, replace `rpn_error` and `rpn_debug_callback`
  with the current `rpn_context` members `error` and `debug_callback` respectively