a list of complimentary variable names for software engineering, symmetric in meaning and size

symmetry.txt

foreground, background
lowerbound, upperbound
inclusions, exclusions
expected, received
incoming, outgoing
hostname, protocol
username, password
abstract, concrete
unknown, missing
minimum, maximum
reading, writing
include, exclude
encoder, decoder
reader, writer
client, server
source, output
source, target
create, update, delete
minion, master
encode, decode
micros, millis
stderr, stdout
status, result, record
since, until
split, strip
inner, outer
lower, upper
field, class, value
major, minor, patch
host, port
base, root
push, pull
next, prev
send, recv
min, max
avg, sum
src, tgt
get, set