Expression providing the confidence value for the correlation item.
Its input (variable named `input`) is a value specific to given matching method,
e.g., Levenshtein edit distance (a non-negative integer) or trigram similarity value
(a decimal number between 0 and 1). Non-fuzzy methods provide the value of 1 there.
It should output a double precision value, usually between 0 and 1, inclusive.
Outputs of such expressions are multiplied together, yielding the final confidence
value for the whole match.