# [VDM-SL], 64 bytes

<!-- language-all: lang-cs -->

    f(a)==forall y in set inds a&card{x|x in set inds a&a(x)=a(y)}=2

[VDM-SL]: https://raw.githubusercontent.com/overturetool/documentation/master/documentation/VDM10LangMan/VDM10_lang_man.pdf