I do, but not for language learning, but for mathematical proofs and logic proofs. Example "Prove that the dot product between 2 unit vectors is cos θ".
For me, this may be too big a card, for what it's worth (depending on how you define the dot product). If it's defined as "|a| |b| cos theta", then fine, but if it's defined as "sum(a_i b_i)", there's definite work involved for me.
