Subsets of a terms object

USAGE:

[.terms(x, i) 

ARGUMENTS:

x
any object inheriting from terms.
i
a subscript, defined on the vector of terms expressions.
This is a method for the function [() for objects inheriting from class terms. The method operates by reconstructing the formula corresponding to the selected subterms, and recreating the terms object. Trying to manipulate the various attributes of the terms object directly is exceedingly error-prone.