1 If X is a topological (or metric) space and M is a metric space, then the set of all bounded continuous functions from X to M forms a metric space if we define the metric as above: d (f, g) = sup x in X d (f (x), g (x)) for any bounded continuous functions f and g.
Invisalign is orthodontic appliance, so since orthodontic treatment is allowed as an FSA expense, Invisalign is covered. Different companies have different rules concerning amounts set aside and the timing of how funds are used for orthodontics.
General Flexible Spending Arrangements (FSAs) will probably make you ineligible for an HSA. If your employer offers a limited purpose (limited to dental, vision or preventive care) or post-deductible (pay for medical expenses after the plan deductible is met) FSA, then you can still be eligible for an HSA.