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Is it possible to define any clause with variables left unbound?

To use an unbound variable as an input argument, it is necessary to declare this argument as a "reference" domain.

(See the Chapter 11 of the "Language Tutorial" (p. 220) and the "Reference Domains Declaration" topic in the online Help).

Note, please, that while declaring a reference variable using standard Visual Prolog domains, you should always declare a special reference domain and use it as locally as possible. For instance:

domains
ref_int = reference integer
ref_int_list = ref_int*

You should not write:

domains
list = reference integer*

since in this case all integers in the module will be treated by the compiler as belonging to the reference domain. The result can be unpredictable - especially in the case of global domains. GPF can be general reaction.