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true. A blockspec evaluated to
trueiff all arguments specified as `-' are unbound.
Multiple BlockSpecs for a single predicate can appear in one or more :- block declarations. The predicate is suspended untill all mode patterns that apply to it are satisfied.
The implementation is realised by creating a wrapper that checks the block conditions and either calls the original predicate immediately (if none of the block conditions were true) or uses attributed variables to delay re-evaluating the block condition until any of the arguments in question are bound.