Assertion: mustard gas | /r/HasAspect | the fact
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Source sentences:
text: Mustard gas used as a weapon had its own set of problems, including the fact that since it was heavier than air and lingered in fox-holes used by opposing troops it remained a blight in those same fox-holes if the frontline advanced and the troops that delivered it in the first place now needed to occupy those same spots to avoid enemy fire.