While there are Questions posted that place explicit restrictions on techniques allowed (and indeed we ask users to give some indication of their backgrounds, so Answers can be tailored to the right level of study), tags are not generally an effective way of "segregating" posts with such restrictions. Even if there were a specific enough tag to ...
Proposal: create qr-decomposition
There are already lu-decomposition, cholesky-decomposition, schur-decompsition and other related tags on eigenvalues or the SVD. But not qr-decomposition, while there seem to be many questions on this.
Alternately, as suggested by Cameron Williams in this (now deleted) question, we might merge all those tags with the tag ...
Proposal: Add the meta tag deleted-comments.
This tag is present on meta.SE: https://meta.stackexchange.com/questions/tagged/deleted-comments.
The current meta tag deletion includes
posts/comments nominated for deletion.
As a result, to search for meta questions about deleted comments, one has to use the ...
I think sometimes, yes, particularly when a more specific tag is context dependent on a broader, more general field.
For example, the tag might be used for a question in geometry, or perhaps in a question in group theory (i.e. symmetric groups), or in a discrete math class when the question might concern symmetric relations.
The tag might concern a a ...