There are several reasons why the execution of a plan may fail, whether internal (the execution of an action fails) or external (changes in the world, damaged or add objects, etc). If we go to temporals plans, the factors may be much larger and more delicate, such action runs faster or slower than expected.

Imagine a non-temporal plan that is running, and in t time is a failure in the
execution of a At action, if we were not able to detect this failure and take necessary measures, the plan could not be left to run.

Detecting a failure involves monitoring at a certain t time each of the variables involved in the plan execution.

For find the variables to be monitored can we employ the Fikes et al., 1972
method called Triangle Table. Triangle Table were designed as lookup tables for operator sequences or macro operators, together with overalls Their effects and preconditions.

With Triangle Table, get the list of variables that must be monitored, this method can be easily extended to temporary plans.