principle layer name, which should not happen (especially in the
case of space, where layers may be aliased to "space" to make them
ignored on input). Also: Implemented a "-<types>" option to the
"substrate" record in the techfile to declare types which shield
layers from the substrate. This allows types like pwell to be used
in different contexts, e.g., as part of the substrate, or as a P-well
in deep N-well, without requiring a different type. This works in
conjunction with the recently-implemented "+<types>" ID types for
devices. All of this may seem unnecessary but helps to reduce the
number of layers needing to be defined, and the subsequent complexity
of the DRC rulesets.
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