label: cis-2-chloro-4-carboxymethylenebut-2-en-1,4-olide <=> trans-2-chloro-4-carboxymethylenebut-2-en-1,4-olide
label_lc: cis-2-chloro-4-carboxymethylenebut-2-en-1,4-olide <=> trans-2-chloro-4-carboxymethylenebut-2-en-1,4-olide