isExpr = isJust . exprPrism
isEqn  = isJust . eqnPrism

exprs :: Foldable t => t (Lang x) -> [x]
exprs = catMaybes . map exprPrism . toList

equations :: Foldable t => t (Lang x) -> [Equation x]
equations = catMaybes . map eqnPrism . toList