How to test that a certain atom is different from the ':' atom?

[josd]

How to test that a certain atom is different from the ':' atom?
When I try the following there is an error

$ scryer-prolog
?- X = abc, X \= ':'.
   error(syntax_error(incomplete_reduction),read_term/3:1).

[UWN]

... and rightly so, because : is defined as an operator and therefore it cannot appear as atom as operand of another operator. Instead of these swindling single quotes, rather use the conforming round brackets. Note that : may still appear without round brackets as an argument in functional notation or as an element of a list.

?- X = abc, X \= (:).
   X = abc.

[triska]

Also note that Scryer Prolog provides dif/2 and dif_si/2, so I would recommend at least using dif_si/2 to denote syntactic disequality of terms. In this way, you get clean errors in cases that cannot be decided at the moment:

?- X = abc, dif_si(X, :).
   X = abc.
?-          dif_si(X, :).
   error(instantiation_error,dif_si/2).

[UWN]

In summary:

?- X=[:,f(:,{:})|:].
   X = [:,f(:,{:})|:].

[josd]

Thanks a lot for the answers and we now use dif_si/2 in https://github.com/eyereasoner/retina/blob/c5d6d7ee5f2c082845cb65fc64722e243511979d/retina.pl#L1094