Example of Soundness & Completeness of Inference Is the following example correct about whether an inference algorithm is sound and complete? 

Suppose we have needles a, b, c in a haystack, and have also an inference algorithm that is designed to find needles.


sound - Only needles a, b and c are obtained.
complete - Needles a, b and c are obtained. Other hay may also be obtained.
You have almost got it right, but your definition of soundness is not quite right, or perhaps too subtle.

I would say that the inference algorithm is sound if everything returned is a needle (hence some needles may be missed) and complete if all needles are returned (hence some hay may be returned too).