processing. Provided one works at a sufficiently abstract level, the control flows in a computation can usually be represented as a finite-state machine, so to complete the X-machine specification all that remains is to specify the processing associated with each of the machine's transitions. The structural simplicity of the model makes it extremely flexible; other early illustrations of the idea included Holcombe's specification of human-computer interfaces,<ref name="Hol88b">M. Holcombe (1988) 'Formal methods in the specification of the human-machine interface', International J. Command and Control, Communications and Info. Systems. 2, pp. 24-34.</ref> his modelling of processes in cell biochemistry,<ref name="Hol86">M. Holcombe (1986) 'Mathematical models of cell biochemistry'. Technical Report CS-86-4, Dept of Computer Science, Sheffield University.</ref> and Stannett's modelling of decision-making in military command systems.<ref name="Sta87">M. Stannett (1987) 'An organisational approach to decision-making in command systems.' International J. Command and Control, Communications and Info. Systems. 1, pp. 23-34.</ref> 1990s X-machines have received renewed attention since the mid-1990s, when Gilbert Laycock's deterministic Stream X-Machine was found to serve as the basis for specifying large software systems that are completely testable.<ref name="HI98">M. Holcombe and F. Ipate (1998) Correct Systems - Building a Business Process Solution. Springer, Applied Computing Series.</ref> Another variant, the Communicating Stream X-Machine offers a useful testable model for biological processes and future swarm-based satellite systems. <ref name="HRRT05"> M. G. Hinchey, C. A. Rouff, J. L. Rash and W. F. Truszkowski (2005) 'Requirements of an Integrated Formal Method for Intelligent Swarms', in Proceedings of FMICS'05, September 5–6, 2005, Lisbon, Portugal. Association for Computing Machinery, pp. 125-133.</ref> 2000s X-machines have been applied to lexical semantics by Andras Kornai, who models word meaning by `pointed' machines that have one member of the base set X distinguished. Application to other branches of linguistics, in particular to a contemporary reformulation of Pāṇini were pioneered by Gerard Huet and his co-workers Major variants The X-machine is rarely encountered in its original form, but underpins several subsequent models of computation. The most influential model on theories of software testing has been the Stream X-Machine. NASA has recently discussed using a combination of Communicating Stream X-Machines and the process calculus WSCSS in the design and testing of swarm satellite systems. Analog X Machine (AXM) The earliest variant, the continuous-time Analog X-Machine (AXM), was introduced by Mike Stannett in 1990 as a potentially "super-Turing" model of computation;<ref name="Sta90">M. Stannett (1990) 'X-machines and the Halting Problem: Building a super-Turing machine'. Formal