The pull request included some changes that were neither accidental nor
unintentional, so only a subset of the changes from pull request #869
submitted by klorpa were manually applied.
behaviour -> behavior
speach -> speech
knowlege -> knowledge
incrments -> increments
stethscope -> stethoscope
staiway -> stairway
arifact -> artifact
extracing -> extracting
The uses of "iff" were left alone.
Close#869
