Explanations through examples

• ?- matches any s-expr
• a matches the atom 'a.
• ?a matches any expression, and binds the variable a to this expression.
• (? integer?) matches any integer
• (a (a b)) matches the only list '(a (a b)).
• ???- can only appear at the end of a list, and always succeeds. For instance, (a ???-) is equivalent to (a . ?-).
• when occurring in a list, ??- matches any sequence of anything: (a ??- b) matches any list whose car is a and last car is b.
• (a ...) matches any list of a's, possibly empty.
• (?x ?x) matches any list of length 2 whose car is eq to its cadr
• ((and (not a) ?x) ?x) matches any list of length 2 whose car is not eq to 'a but is eq to its cadr
• #(?- ?- ???-) matches any vector whose length is at least 2.
• #{foo (?- . ?-) (? integer?)} matches any structure or record foo whose first and second fields are respectively a pair and an integer. You can provide only the fields you want to test. The order is not relevant.

Remark: ??- and ... patterns can not appear inside a vector, where you should use ???-: For example, #(a ??- b) or #(a...) are invalid patterns, whereas #(a ???-) is valid and matches any vector whose first element is the atom a.