toCNFExpression :: FeatureExpression -> FeatureExpression
toCNFExpression (And e1 e2)    = And (toCNFExpression e1) (toCNFExpression e2)
toCNFExpression :: FeatureExpression -> FeatureExpression
toCNFExpression (And e1 e2)    = And (toCNFExpression e1) (toCNFExpression e2)
toCNFExpression (And e1 e2)    = And (toCNFExpression e1) (toCNFExpression e2)