u wish to find out what a context does to a term t, you can
call apply(), which builds a new copy of the term with all of
the instantiations of the context applied to the term t.
But I wrote above that (v3,c1) is a different variable 
from (v3,c2)---what does apply do with uninstantiated variables?
Each context has a unique multiplier (a small natural number);
When apply() gets to an uninstantiated variable v, it returns
a variable with index (multiplier*MAX_VARS)+VARNUM(v).
Of course, this can give you VAR