= Solution
Collect the matching replacements and distinguish all three possible multiplicities:
``
applyRule :: [Rule] -> Char -> String
applyRule rules c =
case [replacement | (key, replacement) <- rules, key == c] of
[] -> error ("no rule for " ++ [c])
[result] -> result
(_:_:_) -> error ("multiple rules for " ++ [c])
``
The <list comprehension> selects precisely the rules whose left-hand character is `c`. A singleton is the required unique match. The third branch also rejects duplicate rules with identical right-hand sides: there are still two matching rules. Finiteness of the rule list makes the uniqueness test decidable. The returned replacement may itself be empty.
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