def sum(self, names):
return Expression({(self, name): <NUM_LIT:1> for name in names})<EOL>
Return the sum of the given names in the namespace.

        >>> v = prob.namespace(name='v')
        >>> v.define([1, 2, 5], lower=0, upper=10)
        >>> prob.set_objective(v.sum([2, 5]))  # v[2] + v[5]