An object representing an executable sequence.
Implemented by SequenceDefinitionInterpreted inheriting from SequenceDefinition, in the Sequences file, for sequences registered at runtime,
and by the generated class inheriting from SequenceDefinitionCompiled inheriting from SequenceDefinition, for sequences already available at compile time.
Embedded execs are employed directly by their rules, have only an EmbeddedSequenceInfo as representation
(without capability of getting called, this occurs only directly in the generated code).