#12 object domain, expr and variable

[Nick Coleman]

creation of a object domain that works like an integer
sparse domain along with expression and variable
classes.

The variables would be used in scenarios like (these are
not set variables:
Var X: {"horse", "cow", "duck", "chicken"}
Var Y: {"horse", "cow", "duck", "chicken"}

X != Y
Y is a member of the set variable BarnAnimals