currently, if a function contains a lambda expression, Clad treats it as a special case of a functor. there's no general support for lambdas with captures yet. however, I believe at this point we should stop treating lambda functions as functors but instead generate separate lambda functions representing the pullback of the initial lambda. this way Clad wouldn't need to modify the lambda class that is impossible for the user to manipulate.

pullbacks of the functions that accept lambdas as their arguments would then need to accept both the initial lambda f and its pullback d_f.

this ensures valid source code generation.