Constraints hierarchy specification needs to be extended to cover method parameter constraints

[jonathan-r-thorpe]

The constraints hierarchy for properties in the Device Model is well described in the current specification.

However, there is no guidance for the implied hierarchy for method parameters.

Each of the parameters can have parameter constraints specified, but are also subject to datatype constraints.

The expectation is that, similar to properties, for method parameters the parameter constraints can override the datatype constraints but must obey the same rules (must fully override, cannot widen the constraints).

This needs stating in the specification.