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====== Rule ======

In order to combine queries and production statements to rules, the XL programming language defines three arrow symbols ''%%==>%%'', ''%%==>>%%'' and ''%%::>%%''. The first two are ''structural rules'' whose
right-hand sides consist of production statements which define a successor graph. The third one is an ''execution rule'' which executes the statement for every match.

Additionally, rules using ''%%==>%%'' extend rules using ''%%==>>%%'' by implicit connection transformations. I.e. with ''%%==>%%'' we can write L-system rules just as we would do this in an L-system environment.

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