Double square brackets

In 112-01-28 I've stumbled upon puzzling double square brackets, that is `[[` and `]]`.

Concept definition in the spreadsheet contains following line:

```
In the first case, the value of a quantity Q is generally denoted {Q}[[Q]] where {Q} is the numerical value and [[Q]] is the unit of measurement.
```

However, the source code of original Electropedia page (https://www.electropedia.org/iev/iev.nsf/display?openform&ievref=112-01-28) has single brackets:

```
In the first case, the value of a quantity Q is generally denoted {Q}[Q] where {Q} is the numerical value and [Q] is the unit of measurement.
```

@ronaldtse What do these double square brackets mean and how to represent them in AsciiMath?

Compare the spreadsheet entry for 112-01-28 with the linked original Electropedia page, focusing on the single versus double square brackets and their AsciiMath meaning. Resolve or document the intended representation so the stored definition matches the source and renders correctly.

Written by the indexing model from the issue text.