# TLA+ Toolbox 1.5.5 release - please update

Hi,

a new TLA+ Toolbox 1.5.5 release has been made available [1][2]. This
release primarily fixes a rare but serious bug that has been in TLC
since its initial implementation.  TLC could generate an incorrect set
of initial states, and hence not examine all reachable states, in the
following situation:

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.

An example 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 == /\ \/ var \in 0..99 /\ var % 2 = 0
\/ var = -1
/\ ...

We advise all users to update as soon as possible.

Thanks
Markus

[1] https://github.com/tlaplus/tlaplus/releases/tag/v1.5.5
[2] https://tla.msr-inria.inria.fr/tlatoolbox/products/