PURPOSE:To give marking to a clause referred to without scanning an entire information area by setting a 2nd marking bit of a clause at 1 when the information on a specific set predicate is produced and handling the clause as that which is always referred to until the time point right before the next executing instruction is accepted. CONSTITUTION:When a garbage collecting function (collecting the undesired areas into a registering code area 200 for clauses forming a name table and a predicate when no unused area is available) functions during the execution of a single instruction, it is handled that a permanent mark (2nd marking bit) also shows a clause which is referred to like a temporary mark (1st mark bit). Then the temporary mark is set at 0 and the permanent mark is kept at 1 as it is after the unused areas are collected. Both an information area and a control stack are idle at the time point when a single instruction ends. Thus only the clauses that can be traced via a name table 100 are kept as they are and all other areas are set at 0 together with the permanent mark also set at 0. Thus the marking can be effected to the clause that is referred to without scanning an entire information area.
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