Labelled Transition Systems

mCRL2 LTS format

An mCRL2 LTS file consist of two sections (although the second section is optional). The first section, starting at offset 0, is an SVC file with type mCRL2 or mCRL2+info. The appendix +info indicates whether states are stored as just numbers or as actual mCRL2 states. The actions of transitions are always stored as a pair of an mCRL2 multiaction and an mCRL2 expression of sort Time. The latter may also be Nil in absence of timing. The precise form of states and transitions is as follows.

State            ::=  STATE(DataExprList)
TransitionAction ::=  pair(MultAct,DataExprOrNil)
DataExprOrNil    ::=  DataExpr | Nil

Optionally, this SVC file is followed by a second section containing information such as a data specification or state parameter names. This section consist of an ATerm in Binary format followed by a 64-bit offset (little endian) and a marker    1STL2LRCm (note the three initial spaces). The offset indicates where this section starts in the file. The ATerm is of the following form.

mCRL2LTS1      ::=  mCRL2LTS1(DataSpecOrNil,ParamSpecOrNil,ActSpecOrNil)
DataSpecOrNil  ::=  DataSpec | Nil
ParamSpecOrNil ::=  ParamSpec(DataVarIdList) | Nil
ActSpecOrNil   ::= := ActSpec | Nil

FSM file format

Files in the FSM file format are accepted as input by several LTS visualisation tools in the mCRL2 toolset.

An FSM file is a human-readable, plain-text file that specifies an LTS. Its contents are of the following form:

FSM ::=  PARAMETERS '\n' '---' '\n' STATES '\n' '---' '\n' TRANSITIONS

containing three sections:

  • The first section specifies the state parameters and their domains;
  • The second section specifies the states of the LTS;
  • The third section specifies the transitions of the LTS.

These sections are separated by lines that contain three dashes: ---.

The format of each of these sections is described separately below.

Parameters section

The parameters section defines the state parameters (or state variables) and their domains of possible values. In every state of the LTS, each of the parameters has one specific value from its domain.

At least one parameter should be specified. Every parameter is specified on a separate line of the following form:


containing the following items:

  • The parameter name: a string of alphanumerical characters;
  • The domain cardinality: a natural number;
  • The domain name: a string of alphanumerical characters;
  • A list of domain values: a space-separated list of quoted values, where every value is a string of characters that does not contain quotes (").

The number of domain values should be equal to the domain cardinality. If the domain cardinality is 0, then the parameter will be ignored. It will not be visible in the tool and the corresponding state values in the states section are ignored.

States section

The states section defines the number of states and the value of every parameter in every state.

At least one state should be specified. The first state specified is taken to be the initial state of the LTS. Every state is specified on a separate line of the following form:


being a list of parameter values: a space-separated list of natural numbers.

The number of parameter values should be equal to the number of parameters defined in the parameters section, including parameters with a domain cardinality of 0.

The ith value in the list specifies the value of the ith parameter of the parameters section in the following way: a value of n specifies that in this state, that parameter has the nth value from its domain. These values are 0-based, meaning that a value of 0 corresponds to the first domain value of that parameter.

Every value should be at least 0 and smaller than the domain cardinality of the corresponding parameter. If that domain cardinality is 0, then the latter restriction does not apply and the value will be ignored.

Transitions section

The transitions section defines the transitions between the states of the LTS.

Every transition is specified on a separate line of the following form:


containing the following items:

  • The source state: a positive natural number;
  • The target state: a positive natural number;
  • The label: a quoted string of characters that does not contain quotes (").

A value of n for either of the states indicates the nth state of the states section. Each of these values should be at least 1 and at most the number of states specified in the states section.



The following FSM file specifies the LTS depicted in the figure above. The state parameter values are indicated next to every state. The state identifiers used in the transitions section of the FSM file are shown inside every state:

b(2) Bool "F" "T"
n(2) Nat "1" "2"
0 0
0 1
1 0
1 1
1 2 "increase"
1 3 "on"
2 4 "on"
2 1 "decrease"
3 1 "off"
3 4 "increase"
4 2 "off"
4 3 "decrease"

Aldebaran format

The Aldebaran file format is a simple format for storing labelled transition systems (LTS’s) explicitly.


The syntax of an Aldebaran file consists of a number of lines, where the first line is aut_header and the remaining lines are aut_edge.

aut_header is defined as follows:

aut_header        ::=  'des (' first_state ',' nr_of_transitions ',' nr_of_states ')'
first_state       ::=  number
nr_of_transitions ::=  number
nr_of_states      ::=  number


  • first_state is a number representing the first state, which should always be 0
  • nr_of_transitions is a number representing the number of transitions
  • nr_of_states is a number representing the number of states

An aut_edge is defined as follows:

aut_edge    ::=  '(' start_state ',' label ',' end_state ')'
start_state ::=  number
label       ::=  '"' string '"'
end_state   ::=  number


  • start_state is a number representing the start state of the edge;
  • label is a string enclosed in double quotes representing the label of the edge;
  • end_state is a number representing the end state of the edge.


The following example shows a simple labelled transition system of the dining philosophers problem for two philosophers, visualised using ltsgraph:


This transition system is represented by the following Aldebaran file:

des (0,12,10)
(0,"lock(p2, f2)",1)
(0,"lock(p1, f1)",2)
(1,"lock(p1, f1)",3)
(1,"lock(p2, f1)",4)
(2,"lock(p2, f2)",3)
(2,"lock(p1, f2)",5)
(6,"free(p2, f2)",8)
(7,"free(p1, f1)",9)
(8,"free(p2, f1)",0)
(9,"free(p1, f2)",0)


The Aldebaran format is originally used in the CADP toolset. To be fully compatible with the original syntax definition, the labels of the edges should consist of at most 5000 characters.