Support for parameter variables ("par" keyword)

[h0nzZik]

From NTL reference manual:

A variable can be declared to be a parameter, meaning that its value does not
change during the execution. Parameters are specified by enclosing the declaration
sequence between the keyword par and a semicolon.

[xbauch]

Consequently, exist in number of copies. We could semantically require that this particular arithmetic term only refers to variables declared as parameters, but it would be nice not to require that the number of instances is a concrete number. It would be left for that particular NTS analysis whether or not to have concrete values for parameter variables.