# Feature: `meval` calculator without variables
A <em>simple</em> spec for a calculator based on the `meval` Rust crate.
## Examples with a simple context
### Background
Let's assume a very simple environment.
Given `x` as:
```
5
```
(which is a prime number);
And `y` as:
```
7
```
(which is another prime), the basic arithmetic operations can be demonstrated as follows.
### Example: Addition
When `input` is:
```
3 + x + y
```
Then `result` is:
```
15
```
### Example: Subtraction
When `input` is:
```
(x * 3) - y
```
Then `result` is:
```
8
```
### Example: Multiplication
When `input` is:
```
x * 2 * y
```
Then `result` is:
```
70
```
### Example: Division
When `input` is:
```
(y * 2) / x
```
Then `result` is:
```
2.8
```
## Constant expresions
Since we started a new `h2` section, the context defined under all prior `h2`
section will be reset.
### Example: Empty context
When `input` is:
```
2 * x
```
Then `result` is:
```
cannot evaluate expression: Evaluation error: unknown variable `x`.
```
_Note_: We get this error because the **Arithmetic opeartions** background expires when we enter the **Constant Expressions** section.