Messages that the bridge sends to the client program.
We send all output to the client in discrete messages. The
message format is very simple. It is easy for a client to reliably parse, and
doesn't require us to think about character encoding. Informally, the format
To make this extremely clear: we first print the type, then a space,
then the len, then a newline character, then the contents, then
another newline character.
The type here is a label that describes what kind of message this is.
It matches [A-Z][A-Z0-9_]*. That is, it starts with an uppercase
alphabetic character, and then contains only uppercase alphabetic, numeric, and
The len says how many characters are in contents. It matches
[0-9]+, i.e., it is a base-10 natural number.