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It is evident that expressions like "as has been seen" make reference to something already mentioned before. However, why is there no pronoun? why not, for instance, "as IT has been seen"? On the other hand, what kind of grammatical category does it belong to?

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    The British National Corpus has no examples of "as it has been observed" and 6 of "as has been observed". So I'd say your it version belongs firmly in the subcategory non-standard (adverbial) prepositional phrases. – FumbleFingers Aug 22 '17 at 15:58
  • It's a subordinate clause, which can't stand on its own. – lux Aug 22 '17 at 17:11
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    'As has been seen,' is a pragmatic marker standing apart from the syntax of the matrix sentence. Its purpose is to refocus on previous discourse and to reaffirm the earlier statement/s. 'As we have seen' is a close paraphrase, a non-passive variant. 'Recapping,', 'To recap,', and 'Let me again stress that' are other possibilities; the syntactical form is flexible. – Edwin Ashworth Aug 22 '17 at 22:36
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    Also: as previously stated. – aparente001 Aug 23 '17 at 1:21
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    Or "as is customary", "as far as is known", "as is clear", "as is often the case", etc. – rjpond Aug 23 '17 at 7:46