The Collins dictionary provides the expression 'difficile à fair' and in the randomly generated examples of 'difficile' gives difficile d'atteindre; difficile à réaliser. difficile à modéliser. Is there a rule as to when 'difficile' is followed by 'de' OR 'à' and does this apply to other adjectives such as in the expression 'dur de choisir' which is given as an alternative answer in the exercise.
I'm sure there is a lesson on this but my mind seems to have gone blank regarding it. Thanks in advance.