previously, MASTERSLICE layers would not be added to obstruction
layers made by "-hide". However, an nwell, for example, that
stuck outside of a prBoundary *would* be recorded, which was
inconsistent. Resolved this by allowing MASTERSLICE layers
in the OBS block, but only for layers that are not a substrate
type. NOTE: It may be better to just insist that a MASTERSLICE
layer define an obstruction type in the "lef" section of the
tech file, and treat it like routing obstructions. Alternatively,
one may question whether special obstruction types are needed at
all, as one could simply define an obstruction as a type without
a port label.
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