Bounded Linear Temporal Logic (LTL) operators. For a bound `n`

, a property
`p`

holds if it holds on the next `n`

transitions (between periods). If ```
n
== 0
```

, then the trace includes only the current period. For example,
```
always 3 p
```

holds if `p`

holds at least once every four periods (3 transitions).

Interface: see Examples/LTLExamples.hs You can embed a LTL specification
within a Copilot specification using the form:
```
var
```

where `ltl`

(operator spec)
`var`

is the variable you want to assign to the LTL specification
you're writing.

For some properties, stream dependencies may not allow their specification. In particular, you cannot determine the future value of an external variable. In general, the ptLTL library is probaby more useful.

# Documentation

eventually :: Int -> Spec Bool -> Var -> StreamsSource

Property `s`

holds at some period in the next `n`

periods. If `n == 0`

,
then `s`

holds in the current period. We require `n >= 0`

. E.g., if ```
p =
eventually 2 s
```

, then we have the following relationship between the streams
generated:
```
s => F F F T F F F T ...
p => F T T T F T T T ...
```

next :: Spec Bool -> Var -> StreamsSource

Property `s`

holds at the next period. For example:
```
0 1 2 3 4 5 6 7
s => F F F T F F T F ...
next s => F F T F F T F ...
```

Note: s must have sufficient history to drop a value from it.

always :: Int -> Spec Bool -> Var -> StreamsSource

Property `s`

holds for the next `n`

periods. We require `n >= 0`

. If ```
n ==
0
```

, then `s`

holds in the current period. E.g., if `p = always 2 s`

, then we
have the following relationship between the streams generated:
```
0 1 2 3 4 5 6 7
s => T T T F T T T T ...
p => T F F F T T ...
```