## Intent

Requires a condition to not occur until another condition is satisfied

## LTL Template

$(\neg (l_1)) \mathcal{U} p$, where $l_1 \in L$ and $p \in M$

Note that the pattern is general and consider the case in which a robot can be in two locations at the same time. For example, a robot can be in an area of a building indicated as l1 (e.g., area 01) and at the same time in a room of the area indicated as l2 (e.g., room 002) at the same time. If the topological intersection of the considered locations is empty, then the robot cannot be in two locations at the same time and the transitions labeled with both l1 and l2 cannot be fired.

## Examples and Known Uses

If the robot enters location $l_1$, then it should have not visited location $l_2$ before. The trace $l_3 \rightarrow l_4 \rightarrow l_1 \rightarrow l_2 \rightarrow l_4 \rightarrow l_3 \rightarrow ( l_2 \rightarrow l_3)^\omega$ satisfies the mission requirement since location $l_2$ is not entered before location $l_1$.

## Relationships

The LTL formula used to encode Past Avoidance Pattern is an example of Avoidance Pattern that ensures a condition to not occur until another condition occurs.

## Büchi Automaton representing accepting sequences of events

where circled states are accepting states and states with an incoming arrow with no source are initial states. The automaton above is deterministic.

## CTL Template

$\forall (\neg (l_1)) \mathcal{U} p$, where $l_1 \in L$ and $p \in M$