Is there an easier way to get a value instead of

' arg1 can contain a number, a symbol
Dim arg1 As Term() = Decision.Preprocessing(args(0), store)
Dim t As TDouble

t = New TDouble(CDbl(arg1(0).ToString))
' Do something to t
answer.AddRange(t.ToTerms())
result = answer.ToArray()
Return True

Are you need to convert array of Term to Double? If so, then use this:
This will return double even if the expression in arg1 is not simplified enough yet.

Thanks.

Is there any documentation of the object model?

There is no systematic documentation of the object model on the wiki at this point. Documentation of the Smath API has really only just began. The sum total of documentation on the API is found here:

Developer Guide

Like you, I had also been thinking that systematic documentation of the object model would be a great asset. However, this would take a fair bit of work. Those of us that are interested in developing plugins need to document what we learn even as we learn it. In this way, the documentation on the wiki will continue to improve.

A great starting point would be to create an overall map of the object model (such as the one seen here for Excel: Excel Object Model). The detailed documentation for the objects, properties, and methods could be slowly added to the wiki over time. But an overall schematic of the object model would provide a framework to build upon.

Andrey, it would be wonderful, when you have time, if you could contribute to our knowledge of the object model. Also, it would be great to see some more demo videos (or tutorial) to demonstrate the use of the IPluginMathNumericEvaluation interface and IPluginMathSymbolicEvaluation interface.

Regards, Will