I laughed

#
# $(1): haystack
# $(2): needle
#
# Guarantees that needle appears at most once in haystack,
# without changing the order of other elements in haystack.
# If needle appears multiple times, only the first occurrance
# will survive.
#
# How it works:
#
# - Stick everything in haystack into a single word,
#   with "|||" separating the words.
# - Replace occurrances of "|||$(needle)|||" with "||| |||",
#   breaking haystack back into multiple words, with spaces
#   where needle appeared.
# - Add needle between the first and second words of haystack.
# - Replace "|||" with spaces, breaking haystack back into
#   individual words.
#