#print
So far all the special characters in the substitute command
have been used in the left-hand expression, the one that
is looked for in the line as it exists before the change.
None of these characters are meaningful in the right-hand
expression; for example,
  s/ab/a./
turn "ab" into "a." just as if "." were an ordinary letter.
But there is one character that is special on the right
side of a substitution, which is '&'.  The ampersand
is replaced by whatever text is being replaced.  In
  s/the/&m/
the string "the" is turned into "them" because the '&' is
replaced by "the".
In this directory there is a file 'text'; on the third line,
change the word 'dog' into 'dogs'; then rewrite the file
and say "ready".
#create Ref
cat
mouse
dogs
rat
#create text
cat
mouse
dog
rat
#user
#cmp text Ref
#log
#next
37.1a 10
37.2a 6