Variables

Variables always start with ?: ?Device, ?X, ?Revision. Without the prefix, a name is a class (after :) or an object (reserve). Quantification is implicit — there are no explicit for all / exists keywords.

// ?X is a variable; Mortal and Human are classes.
?X:Mortal <- ?X:Human.

// ?Revision is a variable the query fills in.
?- reserve[configurationRevision -> ?Revision].