What is the grammar behind "llevar en qué sentarse y con qué alumbrarse"?

Question

I understand the meaning of the sentence:

Con todo, el nuevo Teatro de la Comedia se inauguró cuando todavía no tenía sillas ni lámparas, y los asistentes tenían que llevar en qué sentarse y con qué alumbrarse en los intermedios.

but not the grammar in 'en qué sentarse y con qué alumbrarse'. The correct grammar to me would be 'llevar [las sillas] PARA sentarse y [las lámparas] PARA alumbrarse', where is it documented that 'en qué' and 'con qué' (with tildes) can be used as 'para'?

My second question is: in what category the 'se' is used in 'alumbrarse'? Is that reciprocal? Obviously it is not used to illuminate oneself. I discovered that se can be added to many seemingly arbitrary verbs, not always indicated in a dictionary.

The smaller the words, in this case, the words 'qué' and 'se', the more difficult for a non-native speaker and harder to lookup, therefore I would greatly appreciate your help.

Answer 1

TL; DR:

This is a case of the relative pronoun que with omitted antecedent ("sillas", or "luces"), which makes it stressed (qué).

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This seems to be a modification of the more standard phrase "(no) tener qué + verb". For example,

No tenía qué leer

which means "He did not have anything to read". This example is given in Section of the Nueva Gramática de la Lengua Española (NGLE) as a "uso tónico del relativo qué". In Section 22.1c, such uses are documented:

Pueden ser tónicos o átonos los relativos que incorporan tácitamente antecedentes, como los que se construyen en relativas libres con los verbos tener y haber: [...] No tengo quien me ayude.

i.e., que is a relative pronoun, and when its antecedent is omitted, it can be stressed (have an accent). This means that

• No tenía (nada/ningún libro) que leer

becomes

• No tenía qué leer

by omitting the antecedent "nada" or "ningún libro" and adding an accent to que.

In Section 26.12i, an example of the use of "qué" as a relative pronoun with omitted antecedent (relativo sin antecedente expreso) and introduced by a preposition is given:

una artista como yo no tiene con qué comprar (Vargas, Pasado)

This example is very similar to

• Los asistentes no tienen en qué sentarse

which is formed by omitting the antecedent "sillas" in the sentence "Los asistentes no tienen sillas en (las) que sentarse", and adding an accent to que.

In your example, the same thing happens:

• Los asistentes tenían que llevar en qué sentarse

which, again, is formed by omitting "sillas" in "los asistentes tenían que llevar sillas en (las) que sentarse" and adding an accent to que.

Regarding the use of se in alumbrarse, this is a standard reflexive usage: it just means that the attendees had to bring lights to shed light on themselves. Of course, you can argue that what they really want to shed light on is the theater stage, but it is not uncommon to use alumbrarse to mean "shed light to allow oneself to see". The "oneself" there is the reflexive meaning conveyed by se in Spanish.