As has already been mentioned, in Ada, Part One, Chapter 12, the mention of a hammock begets both these parentheticals:
"(where a former summer guest, with an opera cloak over his clammy nightshirt, had awoken once because a stink bomb had burst among the instruments in the horsecart, and striking a match, Uncle Van had seen the bright blood blotching his pillow)."
"(where that other poor youth had cursed his blood cough and sunk back into dreams of prowling black spumas and a crash of symbols in an orchal orchestra—as suggested to him by career physicians)"
The Darkbloom annotation gives:
“horsecart: an old anagram. It leads here to a skit on Freudian dream charades (‘symbols in an orchal orchestra’).”
Anyhow, I stumbled upon the 'source' for this 'old anagram' in Vladimir Nabokov Interviewed by Penelope Gilliatt (1966):
"At some stage we started to play anagrams. I gave him “cart horse” (the solution is “orchestra”). He took the problem away on what was meant to be a nap, and came bounding into the bar two hours later with an expression that was a very Russian mixture of buoyancy and sheepishness. The tartanned paper of his little notepad was covered with methodically wrong steps. “Her actors,” he said, in try-on triumph, eying me, and knowing perfectly well that the answer had to be one word."
'“Vera has been doing ‘cart horse’ as well,” he said. “Eventually she suggested ‘horse-cart.’ She hadn’t much hope.”'
Fun to note the nap too.