Mercurial - meaning of tokens?

Question

Please see the attached image (courtesy HgInit's Mercurial tutorial).

What does it mean by

1. --- a
2. +++ b
3. @@ -1,4 +1,4 @@

Answer 1

--- a/guac - the outgoing file (the first version to be compared), in this case, since it is about two versions of the same file, a virtual path prefix a/ is prefixed, even when it doesn't exist actually

+++ b/guac - the incoming file (the second version to be compared)

@@ -1,4 +1,4 @@ - the outgoing and incoming line ranges, meaning in this case -1,4 the outgoing/original/first version refers to line 1 and 4 following lines, while +1,4 the incoming/new/second version refers to the same range. Meaning that you'll end up with the same amount of lines (after taking out those with - prefix and putting in those with + prefix, in the rest of the diff chunk)