Return the sum of each name multiplied by a coefficient.

        >>> v = prob.namespace(name='v')
        >>> v.define(['a', 'b', 'c'], lower=0, upper=10)
        >>> prob.set_objective(v.expr([('a', 2), ('b', 1)]))

def expr(self, items):
        """Return the sum of each name multiplied by a coefficient.

        >>> v = prob.namespace(name='v')
        >>> v.define(['a', 'b', 'c'], lower=0, upper=10)
        >>> prob.set_objective(v.expr([('a', 2), ('b', 1)]))
        """
        return Expression({(self, name): value for name, value in items})