set.add and set.delete arguments

[Peter Michaux]

Can set.add and set.delete take multiple arguments? This would go
nicely with the spread of operation on an iterate that was discussed
for multiple arguments to the Set constructor.

var set0 = new Set('alpha', 'beta');
set0.add('gamma', 'delta');
var set1 = new Set('epsilon', 'zeta');
set0.add(...set1);
set0.count; // 6

Peter