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Function ./2
Availability:Arithmetic function (see is/2)
.(+Int,[])
A list of one element evaluates to the element. This implies "a" evaluates to the character code of the letter‘a' (97) using the traditional mapping of double quoted string to a list of character codes. Arithmetic evaluation also translates a string object (see section 5.2) of one character length into the character code for that character. This implies that expression "a" also works of the Prolog flag double_quotes is set to string. The recommended way to specify the character code of the letter‘a' is 0'a.
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LogicalCaptain said (2021-01-05T13:44:32):0 upvotes 0 0 downvotes
Picture of user LogicalCaptain.

Does this still exist? How would one use it?

The operator ./2 is now dict dereferencing, so I guess this "feature" is out?

?- X is 0'a.
X = 97.

But:

?- X is .a.
ERROR: Syntax error: Operator expected
ERROR: X is
ERROR: ** here **
ERROR: .a .

?- X is .[a].
ERROR: Syntax error: Operator expected
ERROR: X is
ERROR: ** here **
ERROR: .[a] .

?- .(X,[a]).
ERROR: Unknown procedure: ('.')/2
ERROR:     However, there are definitions for:
ERROR:         ('.')/3
false.