Editing "piece of plastic" and "landfill"

[iwilkie]

Dear editors,

As part of our work for UN environment on SDG 14 we would like to edit the following terms such that the definitions follow the genus-differentia structure:

Term: piece of plastic
is_a: mass of solid material
New definition: A mass of solid material which is primarily composed of plastic.

Current definition: An abiotic mesoscopic physical object which is primarily composed of plastic.

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Term: landfill
is_a: anthropogenic contamination feature
New definition: An anthropogenic contamination feature which is used for solid waste disposal, filling a natural or artificial depression

Current definition: A site at which refuse is dumped, normally filling a natural or artificial depression.

Note: this definition is slightly more specific based on this article.

I will make a PR regarding this issue.

Thank you!