Prolog - substitute substring of string by letter not used in string itself

Question

I am searching some predicate substitute(+StringInput, +SubString, -Substitution, -SubstitutionLetter), whereby all the substrings identic with +SubString occuring in +StringInput are substituted by a individual symbol not occuring in +StringInput itself. The result is given by -Substition and the individualsymbol is stored in -SubstitionLetter.

e.g.

+StringInput = "(2 + b) + c * 2"  
+SubString = "2"
-Substitution = "(a + b) + c * a"
-SubstitutionLetter = "a"

or

+StringInput= "I went to school on Monday, if I don't get sick."
+Substring = "I"
-Substitution = "a1 went to school on Monday, if a1 don't get sick."
-SubstitutionLetter = "a1"

Answer 1

Note: see https://stackoverflow.com/a/73611612/ for better code.

% Show lists of codes as text
:- portray_text(true).
% Show 100 chars before truncating
:- set_portray_text(ellipsis, _, 100).

% Replace if match found
replace(Find, Replace), Replace --> Find, !, replace(Find, Replace).
% Otherwise accept char-by-char
replace(Find, Replace), [C] --> [C], !, replace(Find, Replace).
% Accept success when reached end
replace(_Find, _Replace) --> [].

Then, can use e.g.:

?- phrase(replace(`I`, `a1`), `I went to school on Monday, if I don't get sick.`, R).
R = `a1 went to school on Monday, if a1 don't get sick.`.

This works if both the find and replace text are provided.

These are lists of codes, i.e.:

?- `ab` = [97,98].
true.