minor fixes

- `use std assert` at the start
- consistently call `assert ...` instead of `std assert ...`
- split "command" and output blocks
- merge `span` and `end` into `span` for the last example

book/testing.md

@@ -2,30 +2,43 @@
 
 ## Assert commands
 
-The foundation for every assertion is the `std assert` command. If the condition is not true, it makes an error. For example:
+Nushell provides a set of "assertion" commands in the standard library.
+One could use built-in equality / order tests such as `==` or `<=` or more complex commands and throw errors manually when an expected condition fails, but using what the standard library has to offer is arguably easier!
+
+In the following, it will be assumed that the `std assert` module has been imported inside the current scope
+```nushell
+use std assert
+```
+
+The foundation for every assertion is the `std assert` command. If the condition is not true, it makes an error.
 
 ```nu
-❯ use std assert
-❯ std assert (1 == 2)
+assert (1 == 2)
+```
+```
 Error:
  × Assertion failed.
  ╭─[entry #13:1:1]
- 1 │ std assert (1 == 2)
- ·  ───┬──
- ·  ╰── It is not true.
+ 1 │ assert (1 == 2)
+ · ───┬──
+ · ╰── It is not true.
  ╰────
 ```
 
 Optionally, a message can be set to show the intention of the assert command, what went wrong or what was expected:
 
 ```nu
-❯ std assert ($a == 19) $"The lockout code is wrong, received: ($a)"
+let a = 0
+assert ($a == 19) $"The lockout code is wrong, received: ($a)"
+```
+```
 Error:
  × The lockout code is wrong, received: 13
  ╭─[entry #25:1:1]
- 1 │ std assert ($a == 19) $"The lockout code is wrong, received: ($a)"
- · ────┬───
- · ╰── It is not true.
+ 1 │ let a = 0
+ 2 │ assert ($a == 19) $"The lockout code is wrong, received: ($a)"
+ · ────┬───
+ · ╰── It is not true.
  ╰────
 ```
 
@@ -34,46 +47,56 @@ There are many assert commands, which behave exactly as the base one with the pr
 For example this is not so helpful without additional message:
 
 ```nu
-❯ std assert ($b | str contains $a)
-Error:
- × Assertion failed.
- ╭─[entry #35:1:1]
- 1 │ assert ($b | str contains $a)
- · ──────┬─────
- · ╰── It is not true.
+let a = "foo"
+let b = "bar"
+assert ($b | str contains $a)
+```
+```
+Error: × Assertion failed.
+ ╭─[entry #5:3:8]
+ 2 │ let b = "bar"
+ 3 │ assert ($b | str contains $a)
+ · ───────────┬──────────
+ · ╰── It is not true.
  ╰────
 ```
 
 While with using `assert str contains`:
 
 ```nu
-❯ std assert str contains $b $a
-Error:
- × Assertion failed.
- ╭─[entry #34:1:1]
- 1 │ assert str contains $b $a
+let a = "a needle"
+let b = "haystack"
+assert str contains $b $a
+```
+```
+Error: × Assertion failed.
+ ╭─[entry #7:3:21]
+ 2 │ let b = "bar"
+ 3 │ assert str contains $b $a
  · ──┬──
- · ╰── 'haystack' does not contain 'a needle'.
+ · ╰─┤ This does not contain 'a needle'.
+ · │ value: "haystack"
  ╰────
 ```
 
 In general for base `assert` command it is encouraged to always provide the additional message to show what went wrong. If you cannot use any built-in assert command, you can create a custom one with passing the label for [`error make`](/commands/docs/error_make.md) for the `assert` command:
 
 ```nu
 def "assert even" [number: int] {
- std assert ($number mod 2 == 0) --error-label {
- start: (metadata $number).span.start,
- end: (metadata $number).span.end,
+ assert ($number mod 2 == 0) --error-label {
  text: $"($number) is not an even number",
+ span: (metadata $number).span,
  }
 }
 ```
 
 Then you'll have your detailed custom error message:
 
 ```nu
-❯ let $a = 13
-❯ assert even $a
+let $a = 13
+assert even $a
+```
+```
 Error:
  × Assertion failed.
  ╭─[entry #37:1:1]