nix3-repl:tldr:ca042

The command :b ${expression} is generally used in scripting or programming languages and it serves as a marker or label for a particular section of code.

In this case, ":b" is the label or marker and "${expression}" is the expression or identifier associated with it. The expression can be any valid variable or value used to identify a specific point in the code.

This command can be useful when you need to refer to a specific section of code later on, for example, when using control flow statements like conditional statements or loops. By labeling a section with :b, you can easily refer back to it without having to write the entire block of code again.

Here's an example in a pseudo-code format:

:b start
   // code block
   // ...
   if condition:
      // code block
      // ...
      goto :b start  // jump back to the ":b start" label
   else:
      // code block
      // ...

In this example, when the condition is met, the program will go back to the ":b start" label and reexecute the code block from the beginning. This can be useful for creating loops or repeating certain sections of code based on certain conditions.