hink of it as a substitution table indexed by the variable number.
<P>
Contexts allow you to separate variables of two terms without
creating a renamed copy of one of the terms.  Say we have
two terms with different Contexts: (t1,c1) and (t2,c2).
If t1 and t2 share a variable, say v3, occurrences in
t1 are a different variable from occurrences in t2, because
the contexts are different.
Think of the those variables as (v3,c1) and (v3,c2).
In fact, when an instantiation is recorded in a Context,
the variable 