Some Prolog systems are able to delay the execution of Prolog goals and resume it later. We were involved in the design, implementation and evaluation of several such systems and we summarise here our experiences with them. First we describe a general structure of such 'coroutining' systems and then we concentrate on the particular features of and choices made in three ECRC's logic programming systems: ECRC-Prolog, SEPIA and ECL'PS e .
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