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AlgebraicRule Class Reference

Detailed Description

Implementation of SBML's AlgebraicRule construct.

The rule type AlgebraicRule is derived from the parent class Rule. It is used to express equations that are neither assignments of model variables nor rates of change. AlgebraicRule does not add any attributes to the basic Rule; its role is simply to distinguish this case from the other cases.

In the context of a simulation, algebraic rules are in effect at all times, t $\geq$ 0. For purposes of evaluating expressions that involve the delay "csymbol" (see the SBML specification), algebraic rules are considered to apply also at t $\leq$ 0. Please consult the relevant SBML specification for additional information about the semantics of assignments, rules, and entity values for simulation time t $\leq$ 0.

An SBML model must not be overdetermined. The ability to define arbitrary algebraic expressions in an SBML model introduces the possibility that a model is mathematically overdetermined by the overall system of equations constructed from its rules, reactions and events. Therefore, if an algebraic rule is introduced in a model, for at least one of the entities referenced in the rule's "math" element the value of that entity must not be completely determined by other constructs in the model. This means that at least this entity must not have the attribute "constant"=true and there must also not be a rate rule or assignment rule for it. Furthermore, if the entity is a Species object, its value must not be determined by reactions, which means that it must either have the attribute "boundaryCondition"=true or else not be involved in any reaction at all. These restrictions are explained in more detail in the SBML specification documents.

In SBML Levels 2 and 3, Reaction object identifiers can be referenced in the "math" expression of an algebraic rule, but reaction rates can never be determined by algebraic rules. This is true even when a reaction does not contain a KineticLaw element. (In such cases of missing KineticLaw elements, the model is valid but incomplete; the rates of reactions lacking kinetic laws are simply undefined, and not determined by the algebraic rule.)

General summary of SBML rules

In SBML Level 3 as well as Level 2, rules are separated into three subclasses for the benefit of model analysis software. The three subclasses are based on the following three different possible functional forms (where x is a variable, f is some arbitrary function returning a numerical result, V is a vector of variables that does not include x, and W is a vector of variables that may include x):
Algebraic:left-hand side is zero0 = f(W)
Assignment:left-hand side is a scalar:x = f(V)
Rate:left-hand side is a rate-of-change:dx/dt = f(W)
In their general form given above, there is little to distinguish between assignment and algebraic rules. They are treated as separate cases for the following reasons:
  • Assignment rules can simply be evaluated to calculate intermediate values for use in numerical methods. They are statements of equality that hold at all times. (For assignments that are only performed once, see InitialAssignment.)

  • SBML needs to place restrictions on assignment rules, for example the restriction that assignment rules cannot contain algebraic loops.

  • Some simulators do not contain numerical solvers capable of solving unconstrained algebraic equations, and providing more direct forms such as assignment rules may enable those simulators to process models they could not process if the same assignments were put in the form of general algebraic equations;

  • Those simulators that can solve these algebraic equations make a distinction between the different categories listed above; and

  • Some specialized numerical analyses of models may only be applicable to models that do not contain algebraic rules.
The approach taken to covering these cases in SBML is to define an abstract Rule structure containing a subelement, "math", to hold the right-hand side expression, then to derive subtypes of Rule that add attributes to distinguish the cases of algebraic, assignment and rate rules. The "math" subelement must contain a MathML expression defining the mathematical formula of the rule. This MathML formula must return a numerical value. The formula can be an arbitrary expression referencing the variables and other entities in an SBML model. Each of the three subclasses of Rule (AssignmentRule, AlgebraicRule, RateRule) inherit the the "math" subelement and other fields from SBase. The AssignmentRule and RateRule classes add an additional attribute, "variable". See the definitions of AssignmentRule, AlgebraicRule and RateRule for details about the structure and interpretation of each one.

Additional restrictions on SBML rules

An important design goal of SBML rule semantics is to ensure that a model's simulation and analysis results will not be dependent on when or how often rules are evaluated. To achieve this, SBML needs to place two restrictions on rule use. The first concerns algebraic loops in the system of assignments in a model, and the second concerns overdetermined systems.

A model must not contain algebraic loops

The combined set of InitialAssignment, AssignmentRule and KineticLaw objects in a model constitute a set of assignment statements that should be considered as a whole. (A KineticLaw object is counted as an assignment because it assigns a value to the symbol contained in the "id" attribute of the Reaction object in which it is defined.) This combined set of assignment statements must not contain algebraic loops—dependency chains between these statements must terminate. To put this more formally, consider a directed graph in which nodes are assignment statements and directed arcs exist for each occurrence of an SBML species, compartment or parameter symbol in an assignment statement's "math" subelement. Let the directed arcs point from the statement assigning the symbol to the statements that contain the symbol in their "math" subelement expressions. This graph must be acyclic. SBML does not specify when or how often rules should be evaluated. Eliminating algebraic loops ensures that assignment statements can be evaluated any number of times without the result of those evaluations changing. As an example, consider the set of equations x = x + 1, y = z + 200 and z = y + 100. If this set of equations were interpreted as a set of assignment statements, it would be invalid because the rule for x refers to x (exhibiting one type of loop), and the rule for y refers to z while the rule for z refers back to y (exhibiting another type of loop). Conversely, the following set of equations would constitute a valid set of assignment statements: x = 10, y = z + 200, and z = x + 100.

A model must not be overdetermined

An SBML model must not be overdetermined; that is, a model must not define more equations than there are unknowns in a model. An SBML model that does not contain AlgebraicRule structures cannot be overdetermined. LibSBML implements the static analysis procedure described in Appendix B of the SBML Level 3 Version 1 Core specification for assessing whether a model is overdetermined. (In summary, assessing whether a given continuous, deterministic, mathematical model is overdetermined does not require dynamic analysis; it can be done by analyzing the system of equations created from the model. One approach is to construct a bipartite graph in which one set of vertices represents the variables and the other the set of vertices represents the equations. Place edges between vertices such that variables in the system are linked to the equations that determine them. For algebraic equations, there will be edges between the equation and each variable occurring in the equation. For ordinary differential equations (such as those defined by rate rules or implied by the reaction rate definitions), there will be a single edge between the equation and the variable determined by that differential equation. A mathematical model is overdetermined if the maximal matchings of the bipartite graph contain disconnected vertexes representing equations. If one maximal matching has this property, then all the maximal matchings will have this property; i.e., it is only necessary to find one maximal matching.)

Rule types for SBML Level 1

SBML Level 1 uses a different scheme than SBML Level 2 and Level 3 for distinguishing rules; specifically, it uses an attribute whose value is drawn from an enumeration of 3 values. LibSBML supports this using methods that work with the enumeration values listed below.