usr/lib/learn/editor/L38.2b

#print
In this directory there is a file "text"; whenever the
letters "cat" begin a line, change them to "mouse";  but
leave other instances of "cat" unaltered.  Do this on each line.
Then rewrite the file and type "ready".
#create Ref
mouse's paw
alley cat
mouse has nine lives
let the cat out of the bag
#create text
cat's paw
alley cat
cat has nine lives
let the cat out of the bag
#user
#cmp text Ref
#log
#next
39.1a 10