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    Pattern: readonly [Variable | Subject, Variable | Predicate, Variable | Object]

    A SPARQL triple pattern, as a subject/predicate/object statement with optional variables.

    Each position generalises the corresponding Triple position by also admitting a Variable: the subject is a variable, an IRI Reference, or a blank node; the predicate is a variable, an IRI, or the "a" (rdf:type shorthand); the object is a variable or any Term. Unlike a ground Triple, a pattern may leave positions unbound for matching. Construct one with pattern.