Compare line by line.

report only when
two are the same

output lines
of unified context

in two columns

each change

show the most recent

expand spaces
make line up

recursively follow
symbolic links

treat absent as empty
ignore differences

ignore changes
due to expansion

ignore end
try hard

assume many scattered
‘never’, ‘always’

if trouble

the complete manual.

Found poem (erasure) from the Linux Man Pages. Complete text here:
Written by Paul Eggert, Mike Haertel, David Hayes, Richard Stallman,
and Len Tower.