Attempting to add an object to an SBMLDocument having a different combination of SBML Level, Version and XML namespaces than the object itself will result in an error at the time a caller attempts to make the addition. A parent object must have compatible Level, Version and XML namespaces. (Strictly speaking, a parent may also have more XML namespaces than a child, but the reverse is not permitted.) The restriction is necessary to ensure that an SBML model has a consistent overall structure. This requires callers to manage their objects carefully, but the benefit is increased flexibility in how models can be created by permitting callers to create objects bottom-up if desired. In situations where objects are not yet attached to parents (e.g., SBMLDocument), knowledge of the intented SBML Level and Version help libSBML determine such things as whether it is valid to assign a particular value to an attribute. For packages, this means that the parent object to which this package element is being added must have been created with the package namespace, or that the package namespace was added to it, even if that parent is not a package object itself.
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