TLA+ Toolbox 1.5.6 release

Hi,

a new TLA+ Toolbox 1.5.6 release has been made available [1][2]. This release fixes an uncommon but serious bug in TLC that has existed since its initial implementation. The bug can cause TLC to generate an incorrect set of initial states, or an incorrect set of possible next states when examining a state. Either can cause TLC not to examine all reachable states. The error can occur in the following two cases:

The possible initial values of some variable var are specified by a subformula F(..., var, ...) in the initial predicate, for some operator F such that expanding the definition of F results in a formula containing more than one occurrence of var, not all occurring in separate disjuncts of that formula.

The possible next values of some variable var are specified by a subformula F(..., var', ...) in the next-state relation, for some operator F such that expanding the definition of F results in a formula containing more than one occurrence of var' , not all occurring in separate disjuncts of that formula.

An example of the first case is an initial predicate Init defined as follows:

VARIABLES x, ...

F(var) == \/ var \in 0..99 /\ var % 2 = 0
\/ var = -1

Init == /\ F(x)
/\ ...

The error would not appear if F were defined by:

F(var) == \/ var \in {i \in 0..99 : i % 2 = 0}
\/ var = -1

or if the definition of F(x) were expanded in Init :

Init == /\ \/ x \in 0..99 /\ x % 2 = 0
\/ x = -1
/\ ...

A similar example holds for case 2 with the same operator F and the next-state formula

Next == /\ F(x')
/\ ...

If your specs are affected by the bug described above, you should re-run TLC on them.

Please consult the changelog for a list of all noteworthy changes [3].

Thanks
Markus

[1] https://github.com/tlaplus/tlaplus/releases/tag/v1.5.6
[2] https://tla.msr-inria.inria.fr/tlatoolbox/products/
[3] https://lamport.azurewebsites.net/tla/toolbox.html#155bug